Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Soft water fish prefer a GH under 5 German degrees of hardness, and as low as 3dGH or less may be better for some, especially for breeding wild caught soft water fish.
Hard water fish prefer a GH over about 9 dGH.
Captive bred fish will usually handle a wider range of parameters than their wild ancestors.
Do enough research about the fish you will be keeping to set the GH at the right level, and to know if they can even be placed in the same water.
Almost all the aquarium plants we grow do not really care what the GH or KH are, as long as they are not zero.
Plants use calcium and magnesium in a ratio of about 4 parts Ca to 1 part Mg, so if you have a Ca test, look up how to do the math and figure out what the Mg is. (The math is not straight forward; it is NOT GH - Ca = Mg)
There are materials available to supplement just Ca or just Mg (Epsom salt) if one or the other is too low. I would only bother with this if I suspected a problem with either Ca or Mg levels.
A blend sold as GH booster is fine in most cases, but read the ingredients. Some contain sodium chloride, which is not the right material for a fresh water tank. Seachem Equilibrium and Barr's GH booster are good products.
Roughly half the aquarium plants we grow can utilize the carbon from carbonates (KH) and do not require CO2 to be added to the tank. Decomposing microorganismsand nitrifying bacteria also use the carbon from KH. Watch the KH so it does not drop below 1 dKH.
It used to be thought that KH under 3 degrees could lead to unstable pH, but many people running tanks with very low KH (Including me) have found that the pH can be quite stable (and quite low) even when the KH barely registers. The nitrifying bacteria are not very active, though, at low pH.
How I do this: My tap water is fairly soft, and already pretty good for soft water fish from hatcheries. Not so good for hardwater fish, or for breeding the most particular of soft water fish.
1) Research the fish. Figure out the optimum GH for the mix of fish in the tank. Start with RO for breeding soft water fish. Tap water for all other fish.
2) Set the GH to suit the fish. I use Equilibrium or Barr's GH booster to raise the GH.
3) Set the KH pretty close to the GH. (plus or minus 1-2 degrees) I use baking soda to raise the KH.
4) Let the pH do whatever it wants.
5) Add peat moss to the filter for black water fish. Add coral sand, oyster shell grit to the filter for hard water fish.