In a recent article,¹ Ron Shimek showed that two salt mixes and natural seawater were more conducive to the development of sea urchin embryos than were two other salt mixes and the water from two reef aquaria using one of them. In that article, Shimek suggested that elevated metal levels, such as copper, could have been responsible. The two salt mixes with poor development are reported to have higher levels of a number of potentially toxic metals than the other two salt mixes or natural seawater.

While there may be other explanations for the observed differences, including ammonia, nitrite, pH, organic compounds present intentionally or as impurities, and the elevation or depletion of many different inorganic chemicals, the hypothesis concerning toxic metals is quite reasonable. As a result of that article, as well as earlier ones that showed the elevated levels of certain metals in reef aquaria and salt mixes,^2,3 many aquarists have become interested in finding out what they can do to lower the toxic metals concentrations in their aquaria. This article starts with the premise that toxic metals are responsible for the biological effects that Shimek observed, and explores what options aquarists have to solve the problem.

Some have assumed, seemingly incorrectly, that if they use natural seawater or one of the salt mixes shown to have lower metals, that the problem will be solved. Unfortunately, some of the things that aquarists need to do in maintaining reef aquaria may well be a primary source of metals. Specifically, the foods and calcium and alkalinity supplements that aquarists use are a big source of metals to reef aquaria. The complication of these inputs may make it difficult, if not impossible, to maintain an aquarium with natural levels of these metals. Both foods and supplements have enough metals in them to take natural seawater to the levels found in the "high metals" salt mixes in a fairly short period of time. In evidence of this possibility, the one aquarium in Shimek's study of reef aquaria that used natural seawater had amongst the highest levels of many of these toxic metals.² Habib Sekha has also indicated that he has obtained similar results on two other aquaria using natural seawater (pers. comm.). That is, the copper and nickel levels were equivalent to those levels found by Shimek in aquaria using artificial salt mixes such as Instant Ocean.² The copper levels in these three aquaria using natural seawater are also comparable to the levels found in my own tank using only Instant Ocean salt mix (10-13 ppb copper).

Additionally, one would expect that the total input of these different metals would be very different between reef aquaria. And yet, all of the aquaria in the study mentioned above had copper concentrations within a factor of two of each other.² That suggests to me that the controlling factor is more likely to be export than import, and this possibility and how it impacts the actions of aquarists wanting to maintain aquaria with low metals is discussed at the end of this article.

In this article, I will focus on those potentially toxic metals that have been found to be elevated in operating reef aquaria,² including copper, nickel, zinc, molybdenum, and cobalt. I will also use the premise that aquarists want to maintain an aquarium with a lower concentration of such potentially toxic metals, and will provide some guidance and general information on how aquarists might usefully go about operating aquaria in that fashion. In some cases, these techniques will relate to many metals, and in some cases they will only apply to certain metals, such as copper.

Using methods described in this article, and other articles to follow that will attempt to further quantify how effective these suggestions are, some aquarists can potentially begin to operate aquaria with much lower levels of such metals. Whether such a reduction will have important advantages that aquarists will appreciate can only be demonstrated when reef aquaria are operated under such low metal concentrations and compared to reef aquaria operated in more traditional means. Once that has been accomplished, then aquarists will be able to look at the results and determine for themselves if this is a necessary, or at least a prudent action for them to adopt.

To begin, however, it is important to note that there are no simple solutions to this potential concern. The two obvious ways to consider lowering the metals concentrations in aquaria are to add less, and export more. There are things that aquarists can do to immediately reduce additions of toxic metals, and these will be detailed in this article. Whether the "normal" export systems (skimming, growth and harvesting of macroalgae, activated carbon, etc.) can be useful may in part depend on how much of an input load for which they are expected to compensate. Other "chemical filtration" systems have been promoted to hobbyists for this purpose, but in the end may not be successful in reducing metals levels below those already present in many aquaria, regardless of the inputs. Still other approaches to export, such as binding metals to calcium carbonate media, are more speculative and will be mentioned as suggestions that will be reported on in more detail in future articles.

In order to understand how and why certain methods of dealing with metals may work (or not), we also need to know the nature of these metals in aquaria. Many aquarists will naturally think of these metals as being like the more familiar calcium ion, floating free through the water on its own. The true situation for many of these metals is, however, much more complicated. It is expected that more than 99.9% of the copper ions present in the water column of reef aquaria are attached to something else, and what they are bound to has a big impact on how they behave. Consequently, before getting into metal removal methods, this article will describe what forms the metals take in seawater, and what forms they are expected to take in reef aquaria.

Metals in Seawater and Aquaria

Different metals take different forms in seawater. Some exist primarily as free metal ions in solution. Of those that aquarists are most familiar with, sodium (Na⁺), potassium (K⁺), calcium (Ca⁺⁺) and magnesium (Mg⁺⁺) fall into this category.⁴ All of these may, to some extent, form complexes with other ions and organic materials, but the free ion tends to predominate in seawater and in typical aquaria.

The metals that we are most concerned about in this article, however, have much more complex behavior. Copper, for example, exists in a multitude of forms.⁴ In natural seawater, it has recently become clear that copper is almost completely bound by organic materials.⁵ Many of these organics are called chelators. A chelating agent is one that can grab onto the copper from two or more directions at once.

In natural seawater, these organics take many forms. Humic and fulvic acids, for example, are two of the most important types of materials that bind copper and other metals in seawater.⁶ They are also known to greatly reduce the toxicity of metals because in many cases it is the free copper that is the most toxic.⁵ These classes of organic materials comprise what remains when proteins, carbohydrates, and many other naturally occurring organic materials are biodegraded to a state where further degradation is very slow. Humic and fulvic acids (the distinction between the two being just that humic acids are more hydrophobic than fulvic acids) have a wide range of structures and physical properties. They typically are high molecular weight organic acids, with sizes ranging from 500 to 10,000 daltons (grams/mole). They can also be parts of larger assemblies of organic materials that would be called colloidal (very small particulates) rather than "dissolved". The humic and fulvic acids are comprised of amino acids, sugars, amino sugars, fatty acids, and other organic functional groups. Different localities and depths in the ocean have different amounts and specific types of these organic materials present. Typical values for the total dissolved organic carbon are on the order of 1 ppm carbon for tropical surface seawater.⁶ Humic substances typically account for about 10-20 % of that total, and fulvic substances can account for more than 50%.⁶

Within these structures will be sites where several carboxylic acid, phenolate, thiolate, amino, or other metal-binding groups come together. These sites are where a metal ion will be most strongly bound. Structurally, it is hard to show a "typical" humic acid binding to copper, but the structure below shows one possibility:

In this figure, the central positively-charged copper ion (Cu⁺⁺) is chelated by the larger humic acid shown in green. It is bound ionically by two negatively charged carboxylic acid groups and complexed by one neutral amino group. Together these three groups may hold the copper ion more strongly by many orders of magnitude more strongly than could any individual binding group.

In the very extensive book "Biogeochemistry of Marine Dissolved Organic Matter",⁶ it is stated:

"It is now widely accepted that the chemical speciation of most bioactive metals in seawater is regulated by strong complexation with natural organic chelators…The cycling of bioactive metals therefor is intrinsic to the behavior of this subset of organic constituents."

And also:

"The collective findings establish that a significant component of bioactive, or nutrient, metals (Mn, Fe, Co, Ni, Cu, Zn, Cd) occur in the colloidal phase along with numerous other trace metals."

Free Metal Ions

In seawater, copper is expected to be bound to organic materials. In one recent study of copper in natural seawater, more than 99.97% was bound to organic materials.⁵ Other metals, such as zinc, may not be as extensively chelated. In aquarium water, where the level of both metals and organics can be higher than in seawater, the percentage bound to organics may be even greater. Nevertheless, unchelated metals are very important. In the case of copper, for example, the unchelated copper ions may represent that portion of the total copper that is toxic to many organisms.⁵ These inorganic forms of copper and other metals are also expected to predominate in freshly mixed artificial salt water that has not been exposed to sources of organic materials.

Assuming that the organism does not take up the entire organic molecule to which the metal is attached (and many humic acids are known to be poorly taken up due to their refractory chemical nature⁶), then chelated metals are often much less toxic than unchelated metals. In seawater, for example, the speciation of copper (i.e., whether and how it is chelated) is often much more important for understanding overall toxicity than is the total copper concentration.⁷

The portions of metals in seawater that are not bound to organic materials are very complicated in their own right. Copper, for example, takes at least 7 different soluble inorganic forms in seawater.⁴ It is comprised of Cu⁺⁺ (3.9% of the inorganic copper), CuOH⁺ (4.9%), Cu(OH)2 (2.2%), CuSO4 (1%), CuCO3 (73.8%), Cu(CO3)2^-- (14.2%) and Cu(HCO3^- )⁺ (0.1%). Similar complications hold for many of the metals that we are concerned with in this article.⁴

Speciation and its Effect on Binding Metals in Aquaria

Since these metals take many different forms in aquaria, one must consider the nature of these different forms when developing methods for removing them. For example, metal ions such as Cu⁺⁺ or Ni⁺⁺ will never absorb at the air/water interface to permit selective removal by skimming. However, if the same metals were bound to an organic material that itself adsorbed to the air/water interface, the metals might well be exported by skimming. Similar concerns relate to claims about metal removal using activated carbon, polymeric ion exchange and complexation resins, and binding to inorganic materials like iron oxides and calcium carbonate. In fact, any proposed method of metal removal will be significantly impacted by the nature of the metal speciation. Depending on what is added to any particular aquarium, the speciation may actually vary from aquarium to aquarium, potentially making generalizations about them less useful.

Further, any experiment that purports to show how well something works must be carried out under conditions of the real speciation present in aquaria. Tests run in artificial salt water (or worse yet, fresh water) are not necessarily of any use in predicting efficacy of metal removal if not carried out in the presence of the typical organics present in aquaria. So when you see products make claims about their ability to remove metals, be skeptical unless you understand what conditions the claims relate to.

Input of Metals: Foods

If the goal is to reduce metals, then looking at the foods that you feed can be important. It will be impossible to eliminate all additions of metals this way, because all marine-sourced materials contain significant amounts of metals that they absorbed when growing in the ocean. Some of these metals are used by the organisms involved, and some are just incidentally accumulated. Nevertheless, there are some things that you might consider when selecting foods if you want low metals.

It turns out that there has been a fair amount of study of many foods for certain metals because they impact human health in various ways. For healthy people looking to ingest adequate copper levels in food, the USDA recommends shellfish, among other things, because they have a naturally high level of copper and zinc.^7,8 Some fish and shellfish may also be unusually high in many metals (including copper, zinc, cadmium, mercury, and lead) because of local pollution in the areas where they are harvested.^9,10

People with Wilson's disease have problems dealing with elevated copper levels, and they need to restrict dietary copper. At a website designed for people with this condition is a table listing the copper levels in many foods, including many that we feed to our aquaria (Table 1).

From this table it is clear that one can select lower copper foods when shopping at the grocery store. Scallops and shrimp, for example, would be much better choices than clams, crab, or lobster. Also, the viscera of squid and crabs contain much more in the way of heavy metals than does muscle tissue.¹⁰ Over the course of a year, these contributions really add up. If you add 5 grams of each of the foods in Table 4 to a 100-gallon aquarium every day, the addition over a year amounts to 178 ppb of copper using lobster and 1.3 ppb of copper using scallops. For comparison, the amount of copper in the salt mixes described in Shimek's article¹ as being high in metals are on the order of 100-200 ppb copper (but only 18 ppb copper in an earlier article²), and those low in copper were 1-40 ppb copper (for comparison, my aquarium using only Instant Ocean salt mix presently has 10-13 ppb copper). So obviously, the choice of foods can potentially make a big impact on the copper levels.

Many aquarists feed commercial foods to their aquaria, rather than fresh seafood. In a study of the amounts of different elements in certain foods,¹¹ Shimek presented the results shown in Tables 2 and 3. While none of these foods appears as high in copper as lobster, lancefish is close and the differences between the various foods are significant. In these tables I have highlighted those values that stand out as unusually high in red and unusually low in green. Bear in mind that some of these foods contain substantial water, and so are naturally more "dilute." For that reason, I included the first line in each table that shows the calories/gram for each food. In this sense, it is easy to see that the "wet" foods are about 4-5 times less concentrated than the dry foods, so in looking at metals, their concentrations need to be multiplied by 4-5 to get equivalent values in terms of actual dosing.

Based on this metals analysis alone, and no other nutritional properties, the Tahitian Blend would seem to be a good overall choice if lower metals were a significant goal (However, Eric Borneman has indicated that Tahitian Blend is a plant material suspension that is of a particle size that will be unusable by many organisms (pers. comm.)). If we knew exactly what specific metal to be most concerned with, the choice might well be different.