How much CO2 does it take to kill fish? I am looking around and can't seem to find a clear answer. I imagine there must be a number of variables that could lead to fish death by CO2, not just the level, but find myself a bit ignorant on this topic... Anyone?

Back story - I was just told that a local AZ hobbyist nuked his fish by adjusting his pH down -0.1 from 7.0 to 6.9 in 7dkH (130ppm kH) water... This was MY advice based on an algae problem he was experiencing, needless to say I feel a bit rotten...

He is managing his CO2 with a pH controller. Is this really possible? I think it is a safe assumption that the tank was not healthy before the change, but from what I understand it was stable at 7.0 (20ppm of CO2) - or at least the fish were not dying...

Thoughts?

According to my drop checker I have occasionallly gone to 40+ ppm of CO2, and the effect was that the fish clustered at the surface - not all of them, but most of them. So, I think it takes more than 40 ppm to kill fish in a brief time. The pH controller will work if you first determine that you have 30 ppm of CO2 in the water, using the drop checker method. Then, measure the tank water pH, and set the controller to maintain that pH. If conditions in the tank change you have to repeat this to maintain the same accuracy.

I am told that the contoller is new and was calibrated with 7.01 and 4.01 solutions... Just a little surprised that given the parameters, a change of -0.1 was the difference between fine and dead. I am thinking there must be more to the story.

I run my CO2 very high in one tank - not sure but must be at least 40ppm - straight up yellow in my drop checker with 4dkh solution.... the tank has been thriving for more than a year.

The important thing with a ph controller, or any other method, is to watch the fish carefully rather than the numbers. If a ph controller is running 24h a day then the fish are most likely to be stressed first thing in the morning just before lights on when the oxygen content of the water is likely to be lowest (some surface movement will help here).

Have a look at this thread:

Co2 adjustment with PH-meter


regards
Neil

Thanks Neil. I am familiar with this technique and personally use a Drop Checker to cross check my levels. Just trying to understand what factors, when put together, lead to death by CO2 injection...

I agree with that. I guess it could be a high enough carbonate hardness such that 7.0 represented a large amount of CO2. The drop to 6.9 could then potentially put the amount of CO2 high enough to kill fish.

It's a stretch though.

I've looked for literature in the past in relation to fish and CO2 tolerance and to honest didn’t find much. What is available will generally relate to aquaculture systems with a lot of work been done on salmonid species that are associated with clean very well oxygenated waters and probably have a lower tolerance than most. I’m a bit pressed for time at the moment but when I get the chance I’ll have another look for this stuff and post anything here that is relevant.

Regards
Neil

I guess before anyone takes the blame here, I would have a few other questions:

Size tank, # and type of fish, Filtration, c02 duration and amount, etc

My ph starts at 7.5-7.6 before c02 injection starts. Ph comes down to 6.6-6.8 on a daily basis, which is much more than +-.01 and the fish suffer no ill effects.

My c02 is between 25-30 ppm and DC color is 6.6-6.8 according to ph color chart....... at 4.0 kh

I have good water current, but NO surface ripple, and the fish never seem stressed. I do have a high fish load as well............

I find it a little hard to believe that the ph swing alone caused this. I totally agree there is more to this story......

A very interesting puzzle though.......

One more angelfish has died.

I have increased the PH to 6.6 (by lowering CO2), withheld any Epson Salt and no Excel, but continued with the fertilizers as usual. I had ordered and installed today a UV sterilizer to help fight my green algae and diatoms, etc. Tired of seeing a cloudy tank. Don't think the UV has had anything to do with the improvement; it's been there too short a time.

There is an almost immediate improvement in all the fish behavior. More active, coming out and moving around, etc.
Three angels have still been at the surface (one of these died) and still wont eat.

One of the angels is mix breed (koi type?) and I can see a new redness in the area of what I would call the lower jaw of lower mouth. Very distinct and clear.
This fish is a little transparent in this area and I, and my wife who first noticed it, can relate this to and irritation of the throat. What I would call a soar, reddish, on both sides. The area is about 1/8 " and kind of circular.

For your opinions, etc

regards,

raul

P.S. The water is clearer now and I can that I am sure the red area mentioned above are the gills, very red.

Ok, some more details:

1) He has been running CO2 in the tank driving the pH to 7.0 since the 5th of October - The tank is 2 months old and the fish have been in there for 7 weeks.
2) 125 Gallon Tank - was all of the Cichlids that died (about 10 of them) - tetras survived but were gasping at the surface
3) He adjusted the pH down to 6.9 on Tuesday evening. At the same time he added more fluorite, which clouded the water up, and added a bunch of fertilzers (not familiar with the products he is using). Thursday morning the cichlids were dead. He did not notice anything strange Wed...
4) Claims that his tap is testing 7.4 to 7.6 - but I live in the same area and the tap is 7.9

I think it may have been a mix of the fact that cichlids have a lower tolerance for this kind of thing, and the additional of flourite, ferts and some additional CO2 all at once... ??? I don't get how they didn't die on Wed morning though...