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Chemistry Mystery

Posted 01/26/2014 at 10:39 AM by zitra

So I have gone through my first chemistry puzzle. I call it "The Vanishing Alk".

As I wrote yesterday, my Alk was coming down hard over the first week of the system life. From the initial 3.66 meq/l to 2.35 meq/l after some six days. While I noted the Ca was very high at around 600 ppm.

After reading numerous pieces of information, understanding the story of CaCO3 precipitation, I have guessed that the high Ca is precipitating in the tank (indeed there is very light white dusting on all the glass and PVC) driving the Alk down in the process.

Yesterday I have added about 13g of NaCO3 and got the Alk up to 3.35 meq/l. Today after some 24 hours the Alk is down to 2.97 meq/l and the Ca is down to 550 ppm.

The rate is supposed to be 20 ppm Ca percipitation gets Alk down by 1 meq/l. My Alk came down in total by 1.69, which should result in 34 ppm Ca drop. I have seen 50 ppm drop, but considering that I am having to extrapolate the Salifert test Ca table I call that close enough.

The Mg is holding little elevated at 1380 ppm.

So if my understanding of all this is correct, then once the Ca comes down to reasonable level somewhere round 420 ppm, the Mg should stop the percipitation and things should balance out. We shall see.

The worry is that I have some 130 ppm of Ca do get down, that is more than 6 meq/l Alk. That sounds like a lot.

The other worry is the Na I am putting into the tank with the NaCO3, but considering the amounts needed compared with the NaCl added in the marine salt I will call that a small worry.

The last one: Where did all that Ca came from? Well there could be a lot of places. Sand in the tank, rocks in the tank, sand mix in the sump. Hopefully it is not the salt (I use the Deep Blue which is a german make recommended by my LFS).
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  1. Old Comment

    Is this a tank that you've just set up? How long has it been running for? What size is your tank? Have you previously dosed anything prior to finding out about this issue?
    Posted 02/09/2014 at 09:30 AM by reefercasey reefercasey is offline

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