I have spent much of the last couple of years taking an indirect look at what happens in reef tanks with regard to various chemical constituents (Shimek, 2001, 2002a, b, c), primarily the toxic heavy metals, referred to in the hobby as “trace elements.” Some of these materials are biologically necessary, but none has been ever shown to have any benefits at concentrations above those found in natural sea water. Indeed, most of them have been shown to be both acutely and chronically toxic at even slightly higher concentrations (see, for example, the discussion in Shimek, 2002d and these references: Alutoin, et al., 2001; Breitburg, et al. 1999; Goh, and Chou, 1992; Heyward, 1988; Negri, and Heyward, 2001; Reichelt-Brushett and Harrison, 1999). Those studies of documented toxicity notwithstanding, I have found that many of these chemicals have exceptionally high concentrations in the liquid medium of reef aquaria, as well as within the food we add to the systems.

There are numerous documentations of presumed cases of metals toxicity in our systems, mostly due to copper contamination. Additionally, there are likely many other cases of “hidden” metals poisoning. Most of these events, from the point of view of the aquarist, likely occur haphazardly and sporadically enough that the actual cause of mortality has either been overlooked or attributed to some other cause. I think that most cases of “acclimation death” are due to insufficient acclimation to the heavy metals concentrations found in our tanks. Most marine organisms can detoxify these materials to one degree or another, but it takes time for the metabolic pathways which can do this (mostly the production of metallothioneins, the proteins used to bind and detoxify metals) to become active. This activation may take from several hours to several days.

Many, if not most, coral reef organisms are from shallow water and, as such, most of them are subject to significant and rapid changes in salinity. Salinity changes of several parts per thousand are not uncommon with tidal shifts and as a result of heavy rains. For example, a few hours of heavy tropical rains can often drop the salinity of a coral reef lagoon from 36 ppt to 30 ppt down to a depth of twenty feet or more. While this stresses the animals, most of them will not die. Likewise, after several days of hot, dry conditions, lagoonal animals often have to contend with salinities of 39 ppt or more. If this occurs during a spring tide period where the tidal exchange is minimal, the salinity may remain elevated for weeks. When the tidal exchanges do become more extreme, such hot saline water flows out over the reef and animals that a few minutes earlier might have had much lower salinity in waters surrounding them. Again this stresses the animals, but it generally doesn’t result in the immediate mortality often referred to in the reef aquarium hobby as “salinity shock.” Frankly, for animals in good health otherwise, transient salinity shock simply is very unlikely to cause any long-term stress. On the other hand, sudden exposure to high concentrations of heavy metals, even if they might be tolerable after acclimation, will be lethal.

I have documented the metal concentrations from samples of aquarium tank water taken from 23 tanks across the United States (Shimek 2002a, b) and are summarized in Table 1, below. These data are unambiguous and conclusive, showing that in these tanks, many of the toxic heavy metals concentrations exceeded demonstrated lethal levels for many corals and other marine organisms (Shimek, 2002d)

There has been much discussion of these results with various proposals being put forth as to why the tanks should not show the effects of toxic metals, even though the metals concentrations were apparently high enough to cause such effects. Some of the explanations for ameliorating effects were:

The metals were simply not toxic.
The metals were being chemically bound in solution by several possible classes of organic chemicals, such as humic acids.
The metals were being bound into some sort of particulate material in the unfiltered water, and the high readings were a spurious result.

All of the ameliorating effects listed may be true to some degree. For example, the metals may not be toxic to some adult organisms even though they are toxic to larvae and immature organism. Toxicity is often demonstrated with larvae or immature organisms as they are known to be more sensitive. Perhaps in some cases, the adults kept in reef aquaria are not as affected as are these other stages. For example, heavy metal toxicity, particularly to adult organisms, is often a long-term process occurring over several years and would not likely be immediately noticeable in many cases, except that “previously healthy” animals die of unknown causes after four or five years in an aquarium system. Those animals that have some method to detoxify the chemicals typically build up a significant body burden of the metals, which, in turn, results in long-term effects such as abnormal behavior, neurological damage, sterility and eventually early death. Additionally, any materials in solution, be they bound to a humic acid, an iron hydroxide, or some other chemical, are ingested by the organisms during their normal feeding (Chong and Wang, 2000; Sundelin et al., 2001). Digestive processes have the potential to release any bound metals, with net effect of transferring them to the animals. Additionally, many invertebrate organisms including corals, actively absorb organic materials across their outer epithelial surfaces. Such absorption will result in ingestion of these metals bound to the organic molecules. Finally, particulate material in the water is actively consumed by many coral reef animals; most notably for aquarists, many of the corals, tridacnid clams, and other suspension feeders. Finally, the presence of organic material may increase trace metal uptake significantly over what occurs in water with less organic materials in it (Guo, et al. 2001; Vasconcelos, et al., 2001; Wang and Dei. 2001). In effect, the amelioration of metals concentrations by organic binding or precipitation likely simply replaces acute toxic effects with long-term chronic ones.

Actual testing of reef aquarium water for toxic effects has not been done. I hoped to report on the results of some sea urchin larval bioassays in time for this writing, by testing various aquarium waters for toxic effects. However, I have been thwarted in those efforts by Mother Nature. Hurricanes Lila and Isidore impacted the Gulf Coast earlier this autumn, resulting in excessive amounts of fresh water runoff, which in turn caused mass spawnings of the local sea urchins around the facility of my source for sea urchins. As I need “unspawned” sea urchins for the tests, I now have to wait until a different species becomes gravid, probably sometime after the first of the year

Two means exist for rendering tank waters less toxic. The first is the precipitation or removal of the metal from the aqueous milieu. Once the material has been removed from the water, it is not toxic. It has to be in the water for organisms to either absorb it or ingest it. One way this precipitation might happen is biologically, as when corals detoxify the mildly poisonous metal, strontium, by incorporating it into their skeletons. Another way to precipitate the chemicals from the water may happen by strictly inorganic means, such as the precipitation of heavy metals by various iron hydroxides. Additionally, particulate substrates such as those found in deep sand beds, have in their depths anaerobic, sulfide-rich conditions that facilitate the precipitation of many of these poisonous materials as non-toxic sulfide minerals. Deep sand beds significantly favor the precipitation and retention of heavy metal sulfides, and are thus likely a significant factor in rendering tank waters less toxic. Precipitated materials in the deep sand beds accumulate with time, but they are not toxic as long as they remain insoluble in the sediments. If those materials should become soluble, however, they would present serious and acute toxicity problems. Such precipitates may become soluble if:

they are exposed to the aerobic conditions in shallower sediments, or
they are exposed to acidic conditions, such as might happen during a calcium reactor malfunction, or
they are eaten by a deposit-feeding animal, or
nutrient loading of the sediments causes bacterial populations to create a more acidic bed.

Any of these events can cause either acute toxicity, or accelerate chronic poisoning, and the events can vary significantly in duration and effect. To remove or reduce the threat of some catastrophic event happening, and to avoid long-term or chronic poisoning, it is to the aquarist’s advantage to reduce the accumulation of these materials either in the water or in the sediments.

The second way to detoxify tank water is to chemically or physically export the chemical out of the system. Several export methods exist for aquarists, but their effectiveness has hitherto not been evaluated in aquariums. In this article, I will describe some of the results from examining a few of the various ways commonly employed to export materials from aquaria.

Materials and Methods:

In this study I was, through the kind and generous help of several aquarists, able to examine the chemical constituents of four types of export materials. I examined 4 samples of skimmate, the liquid resulting from the foam that makes it through a foam fractionation device. I also examined one sample of skimmer sludge, the greenish-gray or black “mud” from the inside of a skimmer. One other person was scheduled to return some sludge, but had to drop out of the study, so unfortunately there was only one sample of sludge. This material is largely bacterial in composition, but as it is washed away during skimmer cleaning, it is a reasonable material to examine. There were three samples of Caulerpa; two samples of Caulerpa cupressoides, one of Caulerpa racemosa. The results from these algae were pooled for the analyses. Two samples of Xenia of unknown species were processed. I also processed one sample of a Sarcophyton species. However, as this latter species is uncommonly used as an export, I will report on it only as a comparison to the commonly exported soft coral, Xenia.

The samples were analyzed as in the Tank Water Study (See Shimek, 2002a, for details). Precleaned and treated jars were sent to me from the analytical laboratory. I repackaged the jars and sent them to the participants. All samples were collected, frozen and returned to me. I refroze them and then sent them to the laboratory.

Varying amounts of the materials were obtained, and in some cases there was insufficient material present to run all the tests. In these cases, the tests were prioritized. The sample results were reported in “as is” basis, based on the total wet weight of the samples. Tests for metals concentrations and nitrogenous compounds were done on all of the samples. Tests of the conventional nutrients and caloric content, however, were not done on several of the samples of skimmate and one of the Xenia samples. The samples had to be dried for these analyses and skimmate is, apparently, a fairly “weak tea.” Even a liter of skimmate contained insufficient material to perform all the tests. The two Xenia samples were of differing sizes, reflecting the growth rates in the two different aquaria that they originated in. One of the samples was simply too small for all the tests.

After processing and analyses, the data were returned to me for further analyses. It is important to realize that, as with the other tests in this series, the statistics that are presented are simply descriptive. I ran no statistical tests comparing the effectiveness of the various exports because there aren’t enough samples for the results to be meaningful. However, there are plenty of data to examine and discuss in the remainder of this article.

All of the descriptive statistics were calculated using spreadsheets, and are subject to operator error. If you find that I have made an error in calculations, please contact me and I will correct it.

Results and Discussion:

Baseline Conditions

In the discussion that follows, the exports will be discussed in relation to three distinctly different types of water. The first is natural sea water (NSW), with the various metals concentrations as reported in Pilson(1998). It is important for our purposes to realize that these data, as authoritative as they may be, may have little to relevancy to the waters over natural reefs. Relatively little chemical oceanographic sampling has been done over reef areas and, in many cases, what has been done was performed in a manner that makes comparisons to reef tanks dubious. For example, samples taken three meters above a reef samples “reef waters,” to be sure, but how those waters relate to what is happening closer to the reef is largely unknown. Most aquarists sample their reefs in far closer proximity than many oceanographers would dream of doing, and the few samples taken close to reefs show some very significant differences from the water a few meters above them. (see, for example: Stimson and Larned, 2000).

In all marine areas within a few inches of the substrate, the chemical composition of the water largely reflects what the nearby organisms have been doing to it. This is particularly the case with biologically “interesting” chemicals, and these, of course, are precisely the items of interest to aquarists. Reef water data taken a few meters above the reef might be, and has been, legitimately argued to tell little about the reef. Unfortunately, this limits comparisons severely, and I have taken the easy way out and just discussed NSW in the familiar general terms.

There is also the comparison with artificial sea water, and here two different comparisons can be made. The first is to what I refer to as the “average reef tank” water as determined by the Tank Water Study (Table 1 and Shimek 2002 b). These comparisons will be made first. The second comparison is to the average of the artificial sea water mixes described by Atkinson and Bingman (1999) and that comparison will be made subsequently.

Table 1.  Average values of elemental concentrations in natural sea water and from the tank study, showing the “excess” amount of materials in an average tank, assuming the average tank volume of 191.3 liters and a specific gravity of 1.025.  All values are in parts per million (mg/kg).  Blank cells indicate that the data are not available.  Values that are “0.000000” do not indicate a value of zero, but rather indicate the actual value is less than 1 part per trillion (the average concentration is less than 10-12).  The variance measures in the average tank data are the sample standard deviations.  Arsenic has no variance measure in the study as it was only found in one tank.

Element

Natural Sea WaterAverage

Average Tank Values± Variance

 

Excess Concentration inAverage Tank (Difference of Averages)

Excess Mass in Average tank in mg

Aluminum

0.00027

0.140 ± 0.070

0.140

33.88

Antimony

0.000146

0.018 ± 0.007

0.018

3.50

Arsenic

0.001723

0.02

0.018

3.58

Barium

0.01374

0.015 ± 0.008

0.001

0.25

Beryllium

0.000000

Not Detected

 

 

Boron

4.6

3.94 ± 1.42

-0.665

-130.42

Cadmium

0.000079

Not Detected

 

 

Calcium

400

400.4 ± 85.1

0.400

78.45

Chromium

0.000208

Not Detected

 

 

Cobalt

0.000001

0.0002 ± 0.0001

0.000

0.04

Copper

0.000254

0.024 ± 0.005

0.024

4.66

Iodine

0.05076

0.447 ± 0.518

0.396

77.71

Iron

0.000056

Not Detected

 

 

Lead

0.000002

Not Detected

 

 

Lithium

0.1725

0.666 ± 1.462

0.494

96.79

Magnesium

1272

1326 ± 139

54.0

10,591

Manganese

0.000027

Not Detected

 

 

Mercury

0.000000

Not Detected

 

 

Molybdenum

0.00959

0.016 ± 0.017

0.006

1.26

Nickel

0.00047

0.024 ± 0.006

0.024

4.61

Phosphorus

0.0713

0.328 ± 0.745

0.257

50.35

Potassium

380

405.2 ± 61.1

25.20

4,942

Silicon

2.81

1.270 ± 1.30

-1.540

-302.03

Silver

0.000003

Not Detected

 

 

Sodium

10561

10850 ± 1246

289.0

56,680

Strontium

13

6.786 ± 1.69

-6.214

-1,219

Sulfur

884

789.6 ± 68.9

-94.400

-18,514

Thallium

0.000012

0.015 ± 0.005

0.015

2.94

Tin

0.000000

0.095 ± 0.01

0.095

18.63

Titanium

0.00001

0.007 ± 0.001

0.007

1.37

Vanadium

0.001527

0.00002 ± 0.0000

-0.002

-0.30

Yttrium

0.000022

Not Detected

 

 

Zinc

0.000392

0.212 ± 0.021

0.212

41.50

Except for a few elements, the concentrations of elements in the water of an average reef tank, as defined by the Tank Water Study, bear little resemblance to the concentrations of the same elements in NSW. In fact, the relationship between reef aquarium water and NSW outside of a couple of the major ions, such as sodium and calcium, is so tenuous that one could legitimately say there is NO consistent relationship. Reef aquarium water has roughly the same amount of salt and calcium as in NSW, but the concentrations of all other ions differ from the concentrations found in natural sea water. Not only that, but they differ with inconsistent magnitudes and directions (Table 1). Nonetheless, many of the ions are substantially more abundant in reef aquarium water than in NSW, and often these are the ions with the highest potential for toxic effects, such as copper, iodine, nickel, and zinc.

The water found in an average tank, however, is not gathered from some place and added to a tank “as is.” Rather, it is the result of several continuous and simultaneous processes that occur to some initial water volume. The initial water can come from either natural sea water or artificial sea water, or a mixture of the two. Once the tank is filled with water, no matter where or how it originates, the initial tank water is manipulated by both the organisms in the reef aquarium system and the aquarist. Organisms actively alter the water in several ways.

They secrete materials that actively bind to and alter the chemical properties of some of the dissolved constituents of tank water.
Environmental conditions of some of the microhabitats in the tank, particularly in a deep sand bed and within live rock, may facilitate the adsorption and removal of many chemical constituents. Organisms, mostly bacteria, living in these areas are largely responsible for the changes from “normal” tank conditions, and as such are responsible for the changes observed.
Some organisms may actively sequester and hold in their bodies many essential biological materials such as phosphorus, iron, and nitrogen (see, for example, Vasconcelos and Leal, 2001).

Additionally, the aquarist manipulates the levels of these chemicals by adding foods, some chemical additives, making water changes, and by exporting various materials.

What organisms are doing to these chemical balances in any given tank is open to supposition; no numerical data about any organism’s secretion or accumulation of materials are known from aquaria, and precious few of these data are available from natural reef areas. That organisms are manipulating their chemical environment is a given. However, reef aquarium concentrations of many of the dissolved trace elements are so different from actual marine concentrations that it is impossible to even reasonably speculate about what is happening in a tank. Simply put, there are too few data available to generalize.

The manipulations that aquarists make to the chemicals found in the tanks may be estimated, given a couple of baselines, and I have attempted to do just that in previous articles in this series (See Shimek: 2002a, 2002b, and especially 2002c), but prior to this article few data have been available on the various means of exportation of materials. The results of the analyses of these materials are shown in Table 2.

Table 2.  Average values of Export Products.  All values are in parts per million ( mg/kg).  Blank cells indicate that the data are not available; generally because the material was not detected in the sample.   The variance measures in the average tank data are the sample standard deviations.  Values without variance measures indicate only one datum for the element in the particular export product or element.  N = Number of samples.  Colored values indicate either the highest single concentrations, or the highest concentrations within the same variance range.

Element

Export Product

Skimmate; N=4 Average ± SSD

Sludge N=1

Caulerpa sp.; N=3 Average ± SSD

Xenia sp.; N=2 Average ± SSD

Sarcophyton sp. N=1

Aluminum

45.43±76.48

560

38.33±24.91

53.50±4.95

Arsenic

0.470

Barium

0.370±0.621

1.900

0.177±0.060

0.170±0.099

0.42

Boron

4.030±3.932

17.00

6.000±1.838

5.400±2.404

6.600

Cadmium

0.128±0.103

0.890

0.200±0.135

0.325±0.021

Calcium

2207 ± 3734

37000

1743±1136

3350±1344

2700

Chromium

0.190±0.255

0.880

0.090

Cobalt

0.133±0.094

1.200

0.400±0.440

0.190±0.057

0.570

Copper

1.385±1.243

3.700

0.587±0.201

0.515±0.346

0.660

Iodine

18.80±28.86

130

48.67±23.29

135±191

95

Iron

25.09±41.49

180

4.567±1.650

3.950±5.586

2.700

Lead

0.477±0.168

0.463±0.194

0.415±0.035

Lithium

0.340±0.286

1.403±1.195

0.555±0.403

2.400

Magnesium

847±537

2000

797±491

1600±566

1800

Manganese

3.315±6.457

23.00

1.663±1.440

0.340±0.170

Molybdenum

1.295±1.394

11.000

1.380±0.430

2.300±0.990

1.900

Nickel

0.890

8.200

0.400

0.575±0.120

5.800

Phosphorus

37.23±61.91

250

80.33±34.06

370±297

160

Potassium

328±219

480

2550±3161

1075±177

1100

Silicon

24.75±43.53

3.800

12.73±9.92

8.500±12.021

2.200

Sodium

10252±7037

6800

11667±12013

5520±7750

16000

Strontium

35.10±50.36

310

24.33±17.10

28.50±7.778

55.00

Sulfur

662±416

880

800±334

1600±566

1000

Thallium

0.550

Tin

0.178±0.123

0.723±0.372

0.460±0.297

Titanium

0.600

0.273±0.140

Vanadium

0.360

0.445±0.191

0.390

Yttrium

0.050

0.320

Zinc

2.005±2.875

9.900

1.420±1.225

42.10±47.94

1.900


Just about everything in a tank appears to be exported by skimming, albeit in often very low concentrations. Skimmer sludge appears to remove much more material on a per unit weight basis, although both Caulerpa and Xenia also concentrate some of the exportable materials in their tissues. Comparison with Table 1 will show that some items are significantly concentrated when compared to tank waters.

Some aquarists have been concerned that some of their exporting methods will alter the salinity of the tank by preferentially removing salt. This does not appear to be the case. The sodium concentration in skimmate and Caulerpa is effectively the same as in the tank water, while in skimmer sludge and Xenia, it appears substantially lower than in tank water. Since sodium and salt concentrations are correlated, it is unlikely that one can export excess salt by any normal export method. However, as some tank water with salt in it leaves the system with each export mechanism, those that contain more water (such as skimmate) do take out some salt and necessitate the regular replenishment of the salt and salinity.

Possibly of more concern is the export of calcium by all of the exports. Significant amounts of calcium are is found in all of the exports; skimmate contains about 2200 ppm calcium, sludge about 37000 ppm, Caulerpa about 1743 ppm and Xenia about 3350 ppm. Put another way, the analyzed sample of skimmer sludge was 3.7 percent, by weight calcium. Harvesting a pound of skimmer sludge removes about 17 grams, over half an ounce, of calcium from the system. If all of this calcium was present as calcium carbonate, and it is doubtful that it would be, then removal of a pound of skimmer sludge would remove about 42 grams, or about an ounce and a third of calcium carbonate. The average tank in the Tank Water Study had a volume of 191. 3 liters and contained a calcium concentration of about 400 mg/kg. Assuming a specific gravity of 1.025, this means that the average tank contained ((191.3) x (1.025)) or 196.0825 kg of artificial sea water. This water, in turn, contained about (196.08 x 0.4) = 78.4.grams of calcium. In other words, removal of one pound of skimmer sludge, would remove 17/78.4 or about 21.7 percent of the calcium in the sea water of the tank. Skimming enough to build up skimmer sludge can be a major way to remove calcium from a tank. In tanks with continual supplementation of calcium, such losses are like not to be noticeable, but in tanks with periodic supplementation, provided they also are accumulating skimmer sludge, the sludge may noticeably be removing calcium. The bacteria in the sludge, or inorganic precipitates, perhaps of calcium phosphate, trapped in the sludge may be responsible for much of the drop seen in calcium concentrations. .

All aquarists should be concerned about the removal of excess toxic materials, particularly heavy metals and toxins produced by organisms from their systems. The export mechanisms can only remove materials found in water, either as suspended, colloidal or dissolved materials, removed by foam fractionation, or by the same kinds of materials that get removed by organism absorption or ingestion. Materials that have been deposited by precipitation into the system’s sediment or by adsorption on to surfaces, or by incorporation into non-exported organisms are not removed from the system. Even though it appears that many of the toxic heavy metals such as copper, zinc, and nickel would be exported rather rapidly, this may not actually be the case. Again, examining the data from an “average” reef tank (Shimek, 2002c), and using those as a basis for calculation, and using the data in Table 2 as an estimate of export rate, the amount of each export material needed to bring the elemental concentrations in an average tank down to that of natural sea water may be calculated (Table 3). I have converted the amount of exports to pounds, as most of the readers in the United States will be more familiar with that measure. Remember that when we consider liquid measurements, the old saying that, “A pint is a pound, the world around,” and you can convert, for example skimmate volume roughly into pounds.

Table 3.  Concentrations of elements in the various exports, and the amount of each export (in pounds) necessary to bring the concentrations from those found in an “average” reef aquarium to the concentration found in natural sea water (NSW)The “average” tank data (volume = 191.3 liters with a specific gravity of 1.025) from Shimek, 2002c. Excess Mass = The number of milligrams of the element found in the average reef tank above what would be found in a tank of the same size filled with NSW. Lb = Number of pounds of the export necessary to remove the excess.  Values with no variance measure indicate the element was detected in only one sample. ND = Not Detected. Values of 0.00 lb indicate less than one hundredth of a pound would be necessary to remove the excess.

Element

Excess Mass in (mg)

Skimmate;
 N=4;  Average  ± SSD

Lb.

Sludge, N=1

Lb.

Caulerpa,  N=3Average ± SSD

Lb.

 Xenia,
N=2
 Average ± SSD

Lb.

Aluminum

33.88

45.43±76.48

1.64

560.00

0.13

38.33±24.91

1.94

53.50±4.95

1.39

Antimony

3.50

ND

 

ND

 

ND

 

ND

 

Arsenic

3.58

0.47

16.78

ND

 

ND

 

ND

 

Barium

0.25

0.37±0.62

1.47

1.90

0.29

0.18±0.06

3.08

0.17±0.10

3.20

Beryllium

ND

ND

 

ND

 

ND

 

ND

 

Boron

-130.4

Average Tank Concentration Below That Of Natural Sea Water

Cadmium

ND

0.13±0.10

 

0.89

0.00

0.20 ± 0.13

0.00

0.33±0.02

0.00

Calcium

78.45

2,206±3,733

0.08

37,000

0.00

1,743±1,135

0.10

3,350±1,344

0.05

Chromium

ND

0.19±0.25

0.00

0.88

0.00

ND

 

ND

 

Cobalt

0.04

0.13±0.09

0.65

1.20

0.07

0.40±0.44

0.21

0.19±0.06

0.45

Copper

4.66

1.39±0.24

7.40

3.70

2.77

0.59±0.20

17.46

0.52±0.35

19.89

Iodine

77.71

18.80±28.86

9.09

130.00

1.32

48.67±23.29

3.51

135±191

1.27

Iron

ND

25.09±41.49

0.00

180.00

0.00

4.57±1.65

0.00

3.95±5.59

0.00

Lead

ND

0.48±0.17

0.00

ND

 

0.46±0.19

0.00

0.42±0.04

0.00

Lithium

96.79

0.34±0.29

626

ND

 

1.40±1.20

152

0.56±0.40

384

Magnesium

10,591

847±537

27.52

2,000

11.65

797± 491

29.25

1,600±566

14.56

Manganese

ND

3.32±6.46

0.00

23.00

0.00

1.66±1.44

0.00

0.34 ± 0.17

0.00

Mercury

ND

ND

 

ND

 

ND

 

ND

 

Molybdenum

1.26

1.30±1.39

2.14

11.00

0.25

1.38 ± 0.43

2.00

2.30 ± 0.99

1.20

Nickel

4.61

0.89

11.41

8.20

1.24

0.40

25.38

0.58 ± 0.12

17.66

Phosphorus

50.35

37.23±61.91

2.98

250.

0.44

80.33±34.06

1.38

370.± 297

0.30

Potassium

4,942

328±219

33.20

480

22.65

2,550±3,161

4.26

1,075 ± 177

10.11

Silicon

-302

24.75±43.53

-26.8

3.80

-175

12.73 ± 9.92

-52.1

8.50 ± 12.02

-78.1

Silver

ND

ND

 

ND

 

ND

 

ND

 

Sodium

56,680

10,253±7,037

12.16

6,800

18.34

11,667±12,013

10.69

5,520±7,750

22.59

Strontium

-1,219

Average Tank Concentration Below That Of Natural Sea Water

Sulfur

-18,514

Average Tank Concentration Below That Of Natural Sea Water

Thallium

2.94

0.55

11.76

ND

 

ND

 

ND

 

Tin

18.63

0.18±0.12

231

ND

 

0.72±0.37

56.67

0.46±0.30

89.11

Titanium

1.37

0.60

5.03

ND

 

0.27±0.19

11.03

ND

 

Vanadium

-0.30

0.36

-1.81

ND

 

0.45±0.19

-1.46

ND

 

Yttrium

ND

0.05

0.00

0.32

 

ND

 

ND

 

Zinc

41.50

2.01±2.87

45.54

9.90

9.22

1.42±1.23

64.30

42.10±47.94

2.17

When one considers the results in Table 3, one must also consider the time necessary to skim out enough foam to make a pound of skimmate, or to grow a pound of skimmer sludge on the inside of the skimmer column, or to grow a pound of Caulerpa or Xenia. As an example for copper, which has a total excess mass in the average tank of 4.66 mg, it would take 7.4 pounds of skimmate or 2.77 pounds of skimmer sludge or 17.46 pounds of Caulerpa or 19.89 pounds of Xenia to bring the copper concentration back into line with that in natural sea water. So, how long would it take to accumulate a gallon of skimmate, or twenty pounds of Xenia? The calculations in Table 3 represent a tank that is not being fed or having the chemical concentrations altered in any way. Such an assumption is unrealistic, of course, but it does allow an estimation of some export rates. It likely overestimates skimmer efficiency, however, because as the concentration of the various materials drop, they will likely be removed at a lower rate. Additionally, reef aquaria need to be fed, and feeding adds these elements to a system.

The average daily feeding ration for some reef tanks was calculated from the data given in the “Tank Water Study.” That ration may be used along with some estimated export rates to assess the efficiency of the export methodology (Table 4). The water was assumed to be “average artificial sea water” using the average of the artificial sea water mixes measured by Atkinson and Bingman (1999), and the “average tank water” from the “Tank Water Study (Shimek, 2002b, c).” I used the following values for each export:

One pound (= one pint) of liquid skimmate (condensed skimmer foam) produced per day,
One half pound (= one standard measuring cup) of skimmer sludge produced each week,
One pound of Caulerpa grown each week, and
One-quarter pound of Xenia grown each week.

I think they are reasonable estimates. However if you disagree, substitute some other values and recalculate; this table is the result of simple algebra.

Table 4. Tank trace element budget, assuming a tank of net volume Of 297.8 liters with a specific gravity of 1.025.  The tank data and average daily ration are taken from Shimek, 2002c.  All of the export methods are working simultaneously.

A. Exports

Amount Assumed Exported in Pounds Per Week

Skimmate

7 lb/wk

Sludge

0.5 lb/wk

Caulerpa

1lb/wk

Xenia

0.25 lb/wk

B. Results of calculations assuming water made from the average of the salt water mixes tested by Atkinson and Bingman (1999).  Net Daily Reduction = Daily Ration – All Exports.  Difference = The difference between the “Artificial Salt Mix” Tank and Natural Sea Water.

Element

Average Tank Total

 (mg)

Net Daily Reduction

(mg)

After 1 Day Difference

Number of days of all exports necessary to match NSW and the number of pounds of export needed to bring the tank to NSW levels (rounded to the nearest pound).

 

Pounds of Export Necessary

Days       

Skimmate

Sludge

Caulerpa

Xenia

Total

Aluminum

77838

316

77522

245

245

17

34

10

306

Barium

143

1.19

141

116

116

8

16

5

145

Cadmium

84.7

0.70

84.0

120

120

8

17

5

150

Chromium

2549

0.48

2548

5269

5269

369

738

211

6586

Cobalt

466

0.87

465

533

533

37

75

21

666

Copper

733

2.78

730

263

263

18

37

11

328

Iron

367

96.6

271

3

3

0

0

0

4

Lead

797

0.61

797

1301

1301

91

182

52

1626

Lithium

83829

1.04

83774

80379

80379

5626

11253

3215

100473

Manganese

370

12.79

358

28

28

2

4

1

35

Molybdenum

797

7.25

790

109

109

8

15

4

136

Nickel

591

4.57

587

128

128

9

18

5

161

Silver

1080

-0.002

1080

Silver is gradually increasing

Titanium

248.

0.39

248

630

630

44

88

25

787

Vanadium

1007

0.36

1007

2764

2764

193

387

111

3455

Zinc

172

24.96

147

6

6

0

1

0

7

C. Results of calculations assuming average tank water as determined from hobbyist tanks (in Shimek, 2002c).  All other calculations as above.  Empty cells indicate elements not detected in the average tank water.

Element

Average Tank Total

Net Daily Reduction (mg)

After 1 Day Difference

Number of days of  all exports necessary to match NSW and the number of  pounds of export needed to bring the tank to NSW levels

 

Pounds of Export Necessary

Days

Skimmate

Sludge

Caulerpa

Xenia

Total

Aluminum

52.8

316

-264

0.17

0.17

0.01

0.02

0.01

0.21

Barium

4.6

1.2

3.4

3.87

3.87

0.27

0.54

0.15

4.83

Cadmium

 

 

 

 

 

 

 

 

 

Chromium

 

 

 

 

 

 

 

 

 

Cobalt

0.06

0.87

-0.81

0.07

0.07

0.00

0.01

0.00

0.09

Copper

7.33

2.78

4.55

2

1.61

0.11

0.23

0.06

2.01

Iron

 

 

 

 

 

 

 

 

 

Lead

 

 

 

 

 

 

 

 

 

Lithium

203.3

1.0

202.3

144

143.53

10.05

20.09

5.74

179.43

Manganese

 

 

 

 

 

 

 

 

 

Molybdenum

4.9

7.2

-2.3

0.67

0.67

0.05

0.09

0.03

0.84

Nickel

7.3

4.6

2.8

1

0.57

0.04

0.08

0.02

0.72

Silver

 

 

 

 

 

 

 

 

 

Titanium

2.1

0.4

1.7

4

4.43

0.31

0.62

0.18

5.53

Vanadium

0.01

0.36

-0.36

0.02

0.02

0.00

0.00

0.0

0.02

Zinc

64.7

25.0

39.8

2

1.59

0.11

0.22

0.06

1.99

In considering the values in Table 4, remember all of the export methods are acting over the same time, so in Table 4B to export sufficient Vanadium to reduce the “average tank water to levels found in natural seawater, the tank would have to be skimmed for 2764 days, removing 2764 pounds of skimmate, 193 pounds of sludge, 387 pounds of Caulerpa, and 111 pounds of Xenia for a total of 3455 pounds…all from a tank of less than 100 gallons. Obviously, while the data may indicate the relative effectiveness of the filtration methods for the various elements, those data do not reflect reality in those situations were only relatively small amounts of materials may be removed by the export method. Even with this caveat, however, the data from the “average artificial sea water” and the “average tank water” are wildly different. These differences likely reflect the processing of the water constituents by the organisms in the tank. As in actual marine situations, the concentration of the aqueous medium surrounding the organisms is maintained by the organisms with minimal effect by purely physical or inorganic processes. The organisms that modify the water the most are the bacteria and cyanobacteria (and some other microalgae) living in the tank. They may secrete materials that bind with toxic metals and make them insoluble, or by the action of their metabolism, they may lower the oxygen tension within sediments or porous rock resulting in the precipitation of some of the toxic trace metals as sulfide minerals or insoluble iron hydroxides (Booij, et al, 2001; Pichler, et al 1999, Pichler, et al., 2000). The sulfur necessary for such for minerals would be the result of anaerobic metabolism of bacteria. Instead of fearing the small amount of hydrogen sulfide gas dissolved in a deep sand bed, aquarists should rather be thankful it is there as an indicator that copper, zinc, and other toxic materials are being sequestered in the sediments by the anoxic conditions in a deep sand bed. Additionally, sediment bacteria can also be incorporating significant amounts into their own biomass. In fact, this statement is also operative with regard to the free-living bacteria in the water and in water borne particulates, as wells as the mucus, surfaces, and associated particulates of the soft corals and algae. All of these materials may be incorporate toxic materials.

Sediment precipitation notwithstanding, the export mechanisms available in the average system seem capable of removing some of the excess materials relatively efficiently (Table 4C) – as long as the remaining excess is detoxified by sediment dwelling bacteria and algae.

These calculations indicate a couple of other potential problems. Freshly mixed artificial sea water is heavily laden with a tremendous excess of potentially toxic heavy metals. Just how toxic this material is remains open to question, however anecdotal and other evidence from invertebrate embryologists indicates it is significantly toxic (See Strathmann, 1987). If freshly mixed artificial sea water contains some toxic concentrations of some trace metals, these metals will not be detoxified immediately in a tank, and until they are, they will be adding to the cumulative toxic chemical load found in the tank animals. This could occur with each water change.

Additionally, tank sediment beds and the porosity of rocks represent a limited volume for detoxified materials. In essence, the sediment beds and rock porosity has a finite capacity for detoxification. Sooner or later, these volumes will become saturated and toxic heavy metals may begin to accumulate in tank waters, or in portions of the sediment which are at the aerobic/anaerobic boundary. Such boundaries are found in deep sand beds and inside of live rock, and their positions fluctuate in nature and in our tanks, primarily with the input of organic matter (food) to the system. If many toxic metals such as copper have been deposited in these marginal areas and the feeding regime in the tank is altered so that that the depth of the boundary changes, significant amounts of toxic materials may be released.

In natural situations, heavy metal pollution typically results in the deposition of metals in the sulfide-rich anoxic layers deep in the sediments. It is likely that a similar deposition pattern occurs in deep sand beds. Consequently, the anoxic areas of the deep sand bed would be the place where tank waters would be detoxified. Water is slowly moved through these areas by the cumulative motions of the animals in the upper layers of the sand beds. This slow percolation of water results in these areas accumulating organic materials. The bacterial utilization of this organic material results in the elimination of oxygen in the deeper sediments. This, in turn, facilitates the removal of metals from solution. However, the accumulation of organic material in these sediments also results in these anoxic sediment layers getting thicker with time. If this occurs, the level where free oxygen occurs becomes shallower. Most animals cannot tolerate anoxic conditions, and will not penetrate those layers. This chain of events leads to a positive feedback loop, working over extended periods. As organic material builds up in tanks, the anoxic layers become deeper forcing the animals into shallower sediment layers. As this occurs, the animals can pump less water through the deep layers. This reduction in pumping facilitates the increase in thickness of these anoxic layers seen in highly polluted areas – or in old reef tanks. In severely polluted situations in natural ecosystems, the anaerobic sediments may actually start at the sediment-water interface (Rosenberg, 1976). In aquaria, a situation such as this is very unlikely; the system will likely crash before it occurs.

It is tempting to suggest that the solution to this quandary would be simply to vacuum the sediments of all organic materials. However, such vacuuming would destroy the functionality of the sand bed as far as its beneficial aspects of excess nutrient processing and feeding the reef are concerned. However, as an alternative to breaking the tank and sand bed completely down, a thorough vacuuming of the sediment followed by re-inoculation of the sediment fauna must be considered as a viable alternative. It is important to note, however, that such vacuuming might not remove much in the way of precipitated metals. Rather, it would be a way to keep the organic loads from becoming extreme. It may also drastically affect pH, redox, oxygen levels and release lots of rather unpleasant anaerobes into the water column.

Alternatively, maintenance of a highly diverse and densely populated sand bed will help utilize much or all of this excess organic material and thus prevent the accumulation of excess organic material. Using this method of sand bed maintenance requires careful monitoring of the sand bed, and period replenishment of the faunal diversity.

Likewise, changes in the overall tank pH cycle may result in brief transient periods of acidic sediment conditions. During these periods, many of the bound heavy metals may become soluble. This would result in transient exposures to these metals. As heavy metal poisoning is commonly cumulative, the final lethal effects of such brief periods of toxicity may be seen only after several months or years.

Organic Exports:

Of course, export of metals is only part of the story. Aquarists need to worry about the accumulation of excess organic nutrients. In this context, I am considering carbon and nitrogen containing compounds as “organic.” Generally, aquarists focus on the visible aspects of the accumulation of these materials, such as the excessive growth of algae and cyanobacteria or large accumulations of detritus. The invisible aspects of organic accumulation in sediments, as discussed above, however, are probably far more serious.

Unfortunately, the determination of organic exports is considerably more difficult than is the determination of metal exports. This is due to the multiplicity of organic export pathways. Carbon and nitrogen containing compounds enter into metabolic pathways and may be broken down to give byproducts that are gaseous. Such gaseous metabolites can leave the tank through any air-water interface and, as a result, are hard to measure or even estimate.

In this study, there was another complication. I gave the trace metal analyses higher priority than the analyses of these organic materials. Consequently, if the samples were too small for the complete analytical array to be done, the tests that were not done related to the organic factors. This resulted in several incomplete analyses (Table 5).

Table 5. Nitrogenous and Organic Exports.  Data for nitrogenous materials in mg/kg (=ppm).  Empty cells indicate insufficient sample for the test.  The caloric value was measured for only one Xenia sample.

 

 

Average±SSD

Material

Sludge

Skimmate

Caulerpa

Xenia

Nitrate + Nitrite Nitrogen

2

2.40±2.27

107.00±46.68

0.61±0.49

Ammonia Nitrogen

530

78.33±105.57

113.67±45.61

585.00±583.47

Total Kjeldahl Nitrogen

4500

421.00±652.99

1016.67±317.54

8600.00±5392.59

Calories (Cal/gm)

 

 

1460.00±729.93

2500

Fat %

 

0.17±0.09

0.16±0.11

0.55

Moisture %         

79

93.67±5.69

94.00±1.00

86.00±8.49

Total Protein %   

2.6

 

0.60±0.17

4.95±3.61

Aquarists often worry about the removal of nitrogen products from their systems, but it is clear that several of the export methods are fairly efficient at removing nitrogen compounds. On a per weight basis both skimmer sludge and Xenia export significant amounts of nitrogen, probably as protein in bacteria and tissue respectively. Comparisons utilizing carbohydrates are incomplete, but tend to indicate that Xenia was again the most efficient export of these materials.

Export efficiency may be measured in a couple of ways, however, and although on a per weight basis Xenia appears to be the best export mechanism, Caulerpa grows far faster in most tanks and it would accumulate a lot more of the needed export per unit time.

Conclusion:

Prior to doing this study, I was quite convinced that none of the export methodologies that were available to aquarists were very good. I was surprised to find that that is definitely not the case with regard to many of the elements needing export. Foam fractionation, coupled with organism export, decidedly provides ways to remove many elements and to keep them from accumulating, given a normal feeding regime. The problem comes with the initial levels of heavy metals concentration found in artificial salt mixes. Unless these excessive amounts of metals can be exported, they will accumulate and, with the passage of time and associated water changes, will become more potentially troublesome. Heavy metal accumulation is organism mediated with both active and passive processes facilitating it. The accumulation products will likely be located in the sediments and inside the porous aquarium rock. If there is also the accumulation of significant organic material in the sediments over time, this may result in periodic transient or chronic low level releases of toxic heavy metals. Heavy metal poisoning in such situations would typically be a cumulative process, resulting in mortality after several months or years. Because of this, sediment cleansing or replacement every few years coupled with the replacement of porous rock substrates may be necessary to prevent heavy metal poisoning of the aquarium’s inhabitants. Alternatives to this drastic and traumatic treatment might include the use of toxic metal sponges, polyfilters and carbon. All of these treatments may all be more efficient and less potentially hazardous than sand bed trauma, however the efficiency of such processes is really unknown. In other words, more work remains to be done before a satisfactory export methodology is available to reef aquarists.

Acknowledgements:

I wish to acknowledge the participation of Guy Comstock, John Link, Joe Peck, Gene Schwartz, Sandra Shoup, James Waltman and William Wiley, who contributed both funds and samples. Howard Pierce, contributed money to defray the cost of the study additional funds were available from the Tank Water Study, and thus I wish to thank to the donors to that study as well. I thank Eric Borneman and Skip Attix for their comments and thorough reviews.


If you have any questions about this article, please visit my author forum on Reef Central.

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Down the Drain, Exports from Reef Aquaria by Ronald L. Shimek, Ph.D. - Reefkeeping.com