From the start, thank you for your time and consideration. Whilst planning a planted aquarium setup that would include CO2 injection, I stumbled across an article about possible behavioral alterations in coral reef fish exposed to higher levels of CO2 in their environment.

I'm wondering if someone would share their opinions on the use of CO2 injection in planted freshwater aquariums and the effects it might have on the livestock. The assumption is that the tank would be heavily planted with CO2 injection starting an hour before the daily 8-hour lighting cycle, turn off an hour before the light shutdown. Target CO2 level in the tank of 25-35 ppm.

2 Answers 2


If your water has the buffering capacity, and you aren't running the tank at 100 degrees F (37 °C), because water basically can't hold oxygen above the mid-90's °F (above 35 °C) or so, the effect of CO2 is negligible. CO2 does not displace oxygen in the water. With regard to the link, fish that live in the ocean live in an incomparably more stable environment with respect to pH, which is the primary effect of CO2 changes, and as far as I have studied there is no comparable effect on freshwater fish when running CO2 in an aquarium, unless you can't keep a reasonably stable environment, then they suffer. pH is a distant measurement for an aquarium, hundreds of things affect it; stability is the most important aspect of chemistry in an aquarium and by running CO2 only during lighted periods as you suggested, you reduce the chances of pH swings due to the way plants consume CO2 and O2. The one caution I would say is to make sure your water has decent buffering capacity (KH) before using CO2. In tap water this normally is never an issue, it is an issue when trying to run a tank using RO/DI and CO2 at the same time, the lack of carbonate hardness facilitates drastic pH changes with very little CO2 addition.


You have missed an important point, alkalinity = buffering. If you have low alkalinity water you will kill the fish. I killed about 25 swordtails once, didn't even add CO2. In rainwater, the fish themselves added enough CO2 to make it very acidic. (I was not thinking when I put them in there, it was only for several hours). I think the 400 ppm in the atmosphere is enough for good plant growth with plain aeration. And you won't be adding CO2 to the atmosphere.


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