Commercially raised fish can cope with conditions outside those that would harm their wild ancestors but this cannot be taken to extreme. Take mollies as an example. These fish have been commercially bred for countless generations yet put them in soft water and they get a condition called the shimmies.
We must also bear in mind that a lot of the less common fish and the hard to breed fish are wild caught and these fish must have water close to their natural habitat.
The main mineral in hard water is calcium, with some magnesium and trace amounts of other metals.
Soft water fish have evolved in water that contains few minerals. Their bodies have evolved to retain minerals as there are few in the water. Put a soft water fish in hard water and their bodies retain too many minerals so that it fills their their organs with calcium. This does not kill them instantly but it does shorten their lives.
Hard water fish have evolved so that their bodies excrete minerals to stop all those minerals in their water clogging their organs. Put hard water fish in soft water and they still excrete those minerals - but the soft water they are now in can't replace them. So the minerals in their bodies become depleted which does harm them, and quicker than keeping soft water fish in hard water.
Think of the hardness scale as very soft, soft, moderately soft, moderately hard, hard, very hard.
A wild fish that evolved in very soft water would probably be OK in soft, and a commercially bred descendant in moderately soft, but even a commercially bred, very soft water fish would suffer in moderately hard water.