Water hardness is important in several ways, if your water is too soft the PH is likely to fluctuate quickly, what fish need is a steady ph. Moderatley hard water is a good base for keeping most aquarium fish and most fish will acclimatise to this kind of water. Water that is too hard can cause difficulty for some fish at breeding time because the eggs cant develop properly. So unless you have very soft water or very hard water, you should be ok.
Research your fishes needs as there is no 'bad' hardness level for all fish, each has their own preference from very soft in adult discus, to very hard for African Cichlids from the great lakes, or central american cichlids.
Ken
