Interesting. I have read from several sources and been told by numerous people on forums that you shouldnt let your Co2 drop below 30ppm in a planted tank or else run the risk of algae problems. Also, that most fish can handle Co2 up to about 90ppm.Blue said:Count the bubbles although it seems frustrating. 1-2 bubbles per second is often considered to be the exact amount. Or if you have a test kit, 20-30 ppm is a safe level for the fish.