I just did a search on the net and here is an excerpt from an artice I found at
http://www.cs.duke.edu/~narten/faq/chemistry.html
General hardness (GH) refers to the dissolved concentration of magnesium and calcium ions. When fish are said to prefer "soft" or "hard" water, it is GH (not KH) that is being referred to.
Note: GH, KH and pH form the Bermuda's Triangle of water chemistry. Although the three properties are distinct, they all interact with each other to varying degrees, making it difficult to adjust one without impacting the other. That is one reason why beginning aquarists are advised NOT to tamper with these parameters unless absolutely necessary. As an example, "hard" water frequently often comes from limestone aquifers. Limestone contains calcium carbonate, which when dissolved in water increases both the GH (from calcium) and KH (from carbonate) components. Increasing the KH component also usually increases pH as well. Conceptually, the KH acts as a "sponge" absorbing the acid present in the water, raising the water's pH.