there are some fish that need softer water, and others that need hard.
i think the question is what is the difference between KH and DH and GH
(correct me if i'm wrong on any of this please)
GH= general hardness of the water. by testing this, you get an overall idea of how hard your water is.. and since you dont care really what "kind" of hardness it is, that's all most fishkeepers need to know.
KH is basically a measurement of hardness based on carbonate. if you have a specific variety of picky fish that must have hard water, or very soft water, sometimes this will be important to check because it works differently from other minerals (which also make water hard) and the fish that need hard water, need carbonate. the fish that need soft water, can't have carbonate. Most of the fish you find in a store are somewhere in the middle.. and as long as your GH is reasonable, they'll be fine.
The best way to avoid any water hardness whatsoever is to use Reverse Osmosis (RO) water. you can buy a unit or you can buy the water. If you're going to have these "extreme" fish, this is usually recommended. (or if your GH is overboard straight from the tap) but for the most part, most fish can adapt just fine.