There is no need to stop pumping CO2 just because your pH is below 6. New substrate like Soilmaster can do that. I hope you didn't stop your CO2 just because of the pH. You will get one hell of an algae bloom if you did. test your KH and put your parms on here so we can help. Also tell us what you did recently....add hardscape, change substrate, how old is tank...etc.