At what levels does CO2 become toxic?
Adding too much CO2 causes the pH to drop, and it is also toxic to fish at high levels. There is some debate over what represents a safe level. Research from fish biologists and vets suggests that levels over 10 ppm can be dangerous in certain cases. However, some CO2 system suppliers claim that levels of over 100 ppm are safe for neons, 500 ppm levels are safe for mollies and 800 ppm levels are safe for guppies. In my opinion, it’s likely to depend on the chemistry of the water and the fish species. Be careful. Interestingly, at high CO2 levels and low pH levels, the oxygen carrying capacity of fish blood is dramatically reduced due to the Bohr effect. This means that losses can occur even if the oxygen level is above the normal safe limits!
