Isn CO2 harmful to fish?
In high concentrations, CO2 can block the respiration of CO2 from the fishes gills and cause oxygen starvation. Since the gills depend on a CO2 concentration differential between the levels in the blood and the water to transfer gases, high levels in the water will reduce the amount of CO2 that can be transferred. Although different references have wildly varying values for toxic levels, a concentration of below 30 ppm is definitely safe. Plants do best at around 15 ppm of dissolved CO2. It is a common misconception that water can hold only so much dissolved gas and adding CO2 will displace oxygen. This is not true. As a matter of fact, if enough CO2 and light is present to enable vigorous photosynthesis, oxygen levels can reach 120% of saturation. Even at night, when the plants stop using CO2 and start using oxygen, the oxygen levels will stay about the same as a typical non-planted aquarium.