Water hardness and pH usually go together. The harder the water, the higher the pH, usually (not always though). The lower the hardness, the lower the pH. Harder water has a better buffering capacity than softer water. Generally, if you are trying to keep fish that prefer softer water, and thus a a lower pH, than water hardness is important. Fish that can withstand a wide range of pH, can also generally withstand a wide range of hardness.
Hardness also effects fish in osmoregulation. If you know anything about osmosis, water moving from a higher concentration to a lower concentration, than you should understand how this works. Generally, you want to try and match the hardness of the fish, so that the fish doesn't have to work so hard to find that happy balance of nutrients.
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