I'm taking this from the paper that came with my Tetratest test kit.
GH= General Hardness is a measure of the dissolved calcium and magnesium salts. The salts directly influence the metabolism of fish, plants and microorganisms. Water with a high calcium and magnesium salt content is referred to as hard and with a low content as soft.
KH= Carbonate Hardness is determined by the carbonate and bicarbonate hardness. KH and pH values are interdependent. The higher the KH value of the water, the more alkaline it is .. an increased pH. Due to the buffering effect of the high KH value, the pH is stabilized against fluctuations. The lower the KH value the more acid the water, i.e. the pH value is decreased and is less pH stable. There is also the risk of marked fluctuations in the pH values, which are dangerous for fish and plant life.
Now I have a question... what is dH and what is the symbol that looks like a "degree" symbol in the acceptable range of KH measurement? I don't have the correct symbol, but it looks something like this for the acceptable range for freshwater: 3◦-10◦ dH.