The problem is that with CO2 it drops the pH. I added Baking Soda to rasie the KH to 5 but the pH keeps dropping. The CO2 shuts off if it drops below 6.5. On and older tank the pH has stayed around 6.8 to 6.9 with CO2. I'm thinking that the KH gets dissolved too fast in the new tank. I also worry about the phosphate the city adds.