Originally Posted by aokashi
Bought fish off a guy who talked about his experience and research with this stuff, apparently hard water boosts immune systems and helps prevent disease, not sure how true this is, but all the fish I've put into my relatively hard tank has yet to get sick...
This is true.. depending on the fish. If he is keeping hard water fish like rift valley cichlids and livebearers this is certainly true. But with fish that like very soft water like cardinal tetra, the exact opposite is true. Keeping them in very hard water stresses them out because their organs have to work harder to maintain equilibrium. This leads to a short lifespan. Unless his "research" has been published in a peer-reviewed journal, I'm going to stick with seasoned aquarists recommendations and the biology that backs it up.