I think you're right on the spot, that certain fish are just not immune to a lot of micro-organisms that flourish in hard water, and it is where the problem comes from, and not the hardness of the water. But a trigger is needed first to further weaken the fish's immune system.....Improperly set up tank, be it lack of enough oxygenation, insufficient filtration, blocked filters, too small of a tank, too many fish, aggressive fish kept with peaceful fish, insufficient water changes, cheap food filled with fillers and no proper nutrition, too much food, too little food, etc...any of these factors will lead to fish diseases way faster than the hardness of one's water.. What we consider sensitive fish, are fish that are not naturally immune to common fish micro-organisms and bacteria and which are subjected to additional negative factors to further weaken their immune system. If the tank is not right, even in their ideal water the fish will still succumb to diseases... I am not familiar with the physiological processes happening when a black-water fish is kept in hard water. I can only say from my own experience of keeping 3 types of black-water fish in my hard water that there are a lot more important factors to their well being than how hard the water is.....I'd say people may be finding excuses in the hardness of their water when they fail to keep fish alive....I know you're familiar with my fish arsenal but I keep clown loaches, kuhli loaches and harlequin rasboras...They don't show any ill effects and neither of them have fallen sick. Oldest are 6-7 years old now(clown loaches and kuhli loaches) and healthy. I hope they remain so for a long long time....However, it will not be my water that would kill them prematurely, but the lack of fresh water....or the lack of proper care on my part, or an accident of some sort. I think one should keep the fish that inspire them to be a fish keeper, and take great care of them with clean water and suitable setup, whether soft of hard water...is not that relevant for majority of fish.