Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If your tap water has GH and KH closer to what you want for the fish, then do a really big water change (like 90%) and see what that does, and how long it stays stable.
If you have some of the same materials you used left over from setting up the tank test these in a jar of water. Put a handful of substrate in a jar of water, put a chunk of sandstone in another jar, and if there is anything else you are adding to the water put some of that in other jars.
Test GH, KH, pH and TDS before you start, then every few days for a week or two, or until you see a change. If the tests run out a couple of weeks with no change, then you can assume that item is neutral as far as the aquarium is concerned.