This is very interesting and clarifying! If I correctly grasped the idea here, my soft water (if not altered) is better suited for soft water fish from south america and south asia (tetras, gouramis, etc) while hard water species such as livebearers need harder water. This totally make sense to me now and gave me an idea of adding a second aquarium that will be dedicated to hard water species (of course adding crushed coral to make the water hard for these guys). A third aquarium will be for saltwater fish like clown fish! I am very excited about this ''science''
There is more to creating hard water from soft source water. Crushed coral is not a good buffer, you need both calcium and magnesium. This is where calcareous substrates and mineral salt preparations come into play. A substrate composed of calcareous sand, such as you can buy for rift lake cichlids, is good. A marine substrate is fine if it does not contain sodium chloride (common "salt"). In addition, at water changes you would mix the water outside the aquarium, by adding rift lake salts to the tap water to get the desired GH. Marine salts are not good here because they do contain sodium chloride, and no freshwater fish needs this and it does cause issues for the fish.
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