It depends more really on the fish you want to keep. some like softer water. 7 which is neutral, is generally soft enough for most fish. If they need softer water, add a half tablespoon per gallon of aquarium salt(FW aqaurium salt only here, marine salt wont work). If they need harder water, Add a little boiled baking soda or commercial DKH prducts. RO/DI water will work find for FW, and if you do it that way, adjusting the PH as needed, you wont have as many problems with Algae(at first). Most municipal water supplies and some sources of well water, have detectable nitrate/phosphates(algae food). Fish waste naturally breaks down into these things, so whether you have problems later depends on husbandry and filtration.