Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I make sure the mineral level (GH) is right for the livestock.
Then set the KH not too far off that.
Let the pH do what it wants.
Nitrifying bacteria is pretty resilient to changes such as the changing pH when you do a water change. They thrive best in hard, alkaline water, but will do just fine down to GH and KH about 3 degrees. Softer than that there is the chance that the carbonates will run out. Plants or the bacteria themselves may use up the carbonates. If you stay on top of it and simply make sure the GH and KH never hit zero the fish, plants and bacteria will be fine.
The fish need stable mineral levels, not so much pH. They do not mind the pH swings that happen when CO2 is added. Keep the GH in the right range for the fish.
Plants need the calcium and magnesium that we test as GH. Make sure it does not run out. Some plants can use carbonates as a source of carbon, but they only do this when the CO2 runs out, and they do not switch over instantly to that method. In a CO2 injected tank I do not think the plants would use the carbonates. As long as there are enough carbonates for the bacteria I would not worry about it.
It used to be thought the KH had to stay above 3 German degrees of hardness to keep the pH stable. Maybe that is a good number to aim for, and see what happens. Many people are finding their tanks do just fine with 1-2 dKH.
Stable is not rock steady at one value 24/7.
Stable is a not extreme change in value that (in the case of pH) follows a daily cycle.