Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Most aquarium plants are quite adaptable to a wide range of GH and KH.
There are a few that are very specific and demand soft, acidic water.
Roughly half the plants we grow can utilize carbonates as a source of carbon, but they only do that when there is no CO2 available.
Nitrifying bacteria use the carbon from carbonates, so I would not allow the KH to drop to 0 degrees.
I would aim to keep the GH at the right level for the fish or shrimp. Then make the KH fairly close to that.
Soft water fish that are wild caught or you are breeding probably need GH and KH not higher than about 3 German degrees of hardness. Research the livestock to be sure.
Soft water fish that have been bred in captivity for several generations can easily handle GH and KH up to 5 degrees, and often as high as 9 degrees.
Hard water fish that will tolerate a range (Many livebearers, Rainbowfish) usually do best with GH and KH between 5-15.
Hard water fish such as the Rift Lake fish will do better with even higher GH and KH. Match the lake, but this could be 20 degrees or more. (Lake Victoria is not quite so hard water).
For stabilizing the pH and CO2 it used to be thought that 3dKH was a good target, but recently people have been posting that a KH as low as 1 degree is enough to maintain a cycle of pH that is acceptable to the fish. Usually this means (without a controller) that the pH will cycle around 1.0 higher in the afternoon (when the plants have removed a lot of CO2) than in the early AM (CO2 is highest after all night or at least an hour or so of CO2 added). So if the pH changes from about 6.5 to 7.5 and back over a 24 hour period that is just fine.
With a controller that is adding more CO2 as the plants use it, based on the pH of course the pH will stay the same. If there is some variation, though, that is OK. The fish won't mind.