Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Water in the ground has been in contact with the rocks and soil. Whatever minerals are in the ground may get dissolved in the water. Some minerals dissolve easier than others.
KH is a test for carbonates. This is a buffer when it it in the water, and tends to make the pH stable. High KH is often linked to high pH. Low KH allows the pH to do whatever the other minerals or acids in the water make it do.
GH is a test for calcium and magnesium. These are needed for plants and fish, but the optimum level varies. Fish from hard water (Rift Lake Cichlids, many live bearers, many Rainbow fish) thrive in hard water with GH over about 9 German degrees of hardness or about 160 ppm. Soft water fish prefer a GH under about 5 German degrees of hardness, or about 100 ppm. (Numbers are rounded. 1 German degree of hardness = 17.9 ppm).
Almost all the plants we keep are quite adaptable to a wide range of conditions.
I would get the GH right for the fish, make the KH about the same (plus or minus a degree or two) and then see if the pH is anywhere near right. If you are keeping fish from a blackwater habitat (many fresh water rivers in rainforests are like this) then I would filter the water through some peat moss before adding it to the tank.
With really weird tap water I think I would start with RO water, add just enough tap water so nothing is too high , then add minerals to raise the other parameters to what you want.
If the KH is over 200 ppm, and I wanted to keep soft water fish I would try a mix of 25% tap water + 75% Reverse Osmosis, then test the GH.
Add enough GH booster to bring it to about 50-80 ppm.