How do I know if my water is too hard or too soft for my fish?
In a word: Homework.
It's fair enough to say that your local fish shop is likely to use tap water to fill their tanks and on the whole, captive-bred tropical fish are a hardy and adaptable lot. Some groups of fish are rather less forgiving, though and Rift-lake Cichlids and Discus are examples of these more demanding specialists.
By testing the hardness of your tap water you can easily determine its suitability for use and select your fish to suit. Sod's law generally dictates that hobbyists with hard water want to keep soft water fish and vice versa. If that's the case then using RO water can free you from the tyranny of tap water completely and buffers can be used to make the softest water suitable for even those Mexican natives that are used to swimming in liquid rock!