Is CO2 gas dangerous for aquarium fish?

It can be harmful for animals in large enough quantities if (1) CO₂ causes the water pH to drop too quickly or (2) people try to be so efficient with the CO₂ that they end up cutting off the oxygen that fish need to breathe. In the latter case, some hobbyists try to minimize surface agitation so that less gas exchange occurs and less CO₂ escapes the water. However, less gas exchange also means less oxygen will enter the water, which can cause your fish to struggle and gasp for air. Our recommendation is to increase both CO₂ and O₂ in the water by using an air stone (or other device that agitates the water surface) in conjunction with your pressurized CO₂ system. Yes, you may have to increase your bubble rate a little to compensate for the slight loss of CO₂, but having enough oxygen for your fish (and plants at night) is more important and can help lead to the pearling effect that is so desired by planted tank enthusiasts.

For more information on CO₂ systems in high tech planted tanks, read the full article.