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Re: Trouble with CO2

John Russo Jr asked the following query:
> A few weeks ago I asked members of this group for advice on raising the KH
> and using CO2 injection to adjust the pH of my freshwater tank (KH =1). I
> keep discus and angels and have recently tried to grow plants. Normally, the
> KH of the water is 1, so I was limited in how low I could get the pH using
> CO2. When I raised the KH to 3 and turned up the CO2 the fish showed extreme
> O2 deprivationčrapid breathing at the surface until exhausted. I never did
> get the pH to less than 7.4. After a few days the membrane diffuser clogged,
> so I replaced it with an air stone, but could not get the pH below 7.4 (most
> of the time it was higher). I lost 2 baby discus and one of my angels almost
> died. All the fish had difficulty breathing. Facing apparent disaster, I
> turned off the CO2 and increased the air bubbles to increase the O2 level.
> Today, the fish (those that didn't die) look better.

I would like to add my two cents worth if I may. From the information I
have learned from this forum and all of it's amazing specialists, your
targets of kH 3 and pH of about 6.8 are correct. That would give you an
CO2 reading of only 14.0 ppm according to "The Chart". I am sure,
though, that there is something else at play in your tank causing the
CO2 to go so high that the fish are under stress. First thing I would
check is whether you are using any other substances in the tank which
raise pH.

I feel qualified to answer this question because I had a similar problem
trying to keep pH down because my plants were taking up CO2 so well. I
thought if I just lowered pH with an acid, that would do the trick.
Well, this meant that any readings I took of pH and kH for calculating
CO2 would result in a CO2 reading that was completely wrong. In my case,
much less CO2 than I intended was actually being delivered. So check for
other pH buffers and remove them.

Next, I would replace the pH test kit just to make sure that it is
reading correctly. A false reading here can deliver way more CO2 than
you intend. Finally, if that does not do it, replace the kH test kit for
the same reason. If the kH is reading much lower than is really there,
then you will have delivered way too much CO2. By the way, what did you
use to raise the kH? Sodium bicarbonate is the most popular choice, but
Potassium carbonate is a good alternative as well (Thanks Tom Barr for
that suggestion!).

Hope this helps! Don't give up yet. It is worth the trials.

Ed Dumas