Ph drop confusion for co2 injection


New Member
Oct 16, 2023
Hi, I’m having a bit of a issue or confusion

PH of tank water degassed with air stone in a cup 7.7
PH of tank water in tank aerated with air stone 6.9 won’t go above this even after fresh water changes

using tropica aqua soil which I believe is the issue

my question is what should I drop the ph to when injecting co2 6.7 or 5.9

many thanks



Super Moderator
Staff member
Lifetime Member
Jun 20, 2016
My suggestion would be to start at 6.7 and slowly go down while monitoring the fish behaviour. That is the safe way rather than risking exposing them to deadly concentrations. A drop checker with suitable 4dKH liquid can serve as a sanity check. It may not have the precision of a pH probe but it will only be affected by the CO2 in the water and not other acids (from the soil). If that is very light green and stays there at 6.7 you are good. The pH probe is still very valuable in monitoring how stable your CO2 is throughout the day.

Sometimes, especially in well aerated environments, airstones in a cup are very efficient. The original idea was for a glass of water left still overnight. Try that and see where the pH lands