Why do you want to lower your pH? 7.8 is fine for most domestic, captive bred tropical fish in the US. Where are you?Heron said:I have a 300l tank with a selection of fish in it. Recently the pH dropped quickly. I tested the water and found the kh was little above 0 . I used nt labs kh up and raised the kh to 6 over a few days. Now the pH is stable but at about 7.8. can I lower it back to 7.2 without lowering the kh again?.