| | Re: CO2 system suggestions for a 75gl tank
CO2 doesn't drop the pH enough to damage fish or plants. Too much CO2 is hard on the fish, but not because it lowers the pH. A pH controller can actually result in losing fish. If the controller fails to work properly, you will add too much CO2 to the tank and the CO2 will kill the fish. In my opinion they are a waste of money, and a detriment to keeping a good healthy aquarium.