CO2 Poisoning???

[mac524]

I'm still pretty new at this planted tank thing. I have a 120 gal heavily planted tank. It's stocked moderately heavy as well. This morning I found 5 dead fish and the rest of them were hanging near the top looking like they would be leaving soon as well. Because of the gasping, the first thing I did was add aeration with an air stone and I phsically agitated the water as well. The remaining fish started looking better almost immediately. I did some tests and learned that my ph was low and nitrates a little high but not bad as to kill so many fish at once and almost kill the whole tank. Ammonia was zero by the way. The closest thing I could find online that best described what was going on was CO2 poisining.

Yesterday, I did a 30% water change and gave my plants a very long overdue pruning. A very heavy pruning I might add. I'm still not well-versed on this kind of stuff but could that have somehow caused a spike in my CO2 levels? I don't have a CO2 test kit and I'm not supplementing with CO2 either.

[Damon]

Test for your Co2 levels. This can be done by testing your PH, and KH. Once you do this, you can figure out if it was CO2 poisoning or just lack of oxygen.

[tungsram]

If it's truly "heavily planted" then isn't O2 indeficiency a very, very remote possiblilty? Considering all those plants put off O2? I've never used a bubbler in any of my tanks and I've not had a single fish die because of "suffocation". (although i've screwed up many times in other ways)

[TANKER]

From what I am reading my guess would be that you had a PH crash. When your ph crashes your nitrates spike as the ph drops which also makes any co2 in your water that much more toxic as it has no buffer to protect your fish. The lower the ph the more toxic the co2 due to higher concentration in other words. Nitrates in your water act in much the same way as the co2 unfortunately, and when the ph crashed, if that is in fact what happened, the two items together proved fatal.

[Damon]

A PH crash has nothing to do with Nitrates or nitrites. It can happen (although very, very, rarely) when your KH is so low (2 or below) it causes a dramatic PH drop when adding CO2 into a tank. Never heard of one happening with DIY Co2 and only twice with pressuirzed CO2. Its not likely that is was a PH crash. I don't even think its CO2 poisoning in a tank that large.

[DavidDoyle]

Generally, it is not excess co2 but a lack of oxygen that causes fish to suffocate. If you have a decent amount of live plants it is hard to imagine too much co2 or too little O in the water. A sudden drop in pH can stress and kill fish.

the nitrogen cycle produces nitric acid (nitrate). Without buffering, your tank's pH would drop over time (a bad thing). With sufficient buffering, the pH stays stable (a good thing).

from http://fins.actwin.com/aquariafaq.html

Rising nitrates work to lower pH. However, in a well planted tank nitrates should tend to be on the low side unless water changes not being done. When was your last water change? You said you tested for pH and nitrates, did you also test for either ammonia or nitrites? Also is tank temp OK? The warmer the water, the less disolved oxygen it contains.

Sorry for having more questions than answers, but you need to make sure you hit all the bases in trying to figure out what went wrong.

[vollphann]

Hi Mac- I have a 42g custom tank (20lx20wx20h) I started a DIY CO2 system with one 2LT bottle yesterday. Within 4 hrs my pH went from 7.2 to 6.8. Quite a drop. Still not sure how to manage it yet. Let you know when I do.