With hard water you have the advantage of extremely stable water. Look around the amount of issues people have in soft water tanks....Although majority of aquarium fish come from soft water habitats, our soft water tanks are too small to be stable....
As a result one can actually be blessed by having hard water, although on first glance not knowing it because of all the contradicting info on the internet....
Many of the commercially bred fish will have no problem in hard water. I personally have wild caught species, e.g. clown loaches, denison barbs, kuhli loaches, hill stream loaches, etc..I also have harlequin rasboras and corydoras. None of these fish come from hard water habitats...
I haven't kept Amazonian fish(besides corydoras) but as you can see majority of my fish come from black water habitats in Asia. They're all in clear hard water, with ages varying from 8 years to the youngest additions from a year and a half. They're all very healthy, very colorful and eat eagerly. I haven't had as much as a split fin issue. They may not breed in hard water but most of the species I own have not been easily bred in captivity anyway.
If you want something beautiful, interesting and small that does come from hard water, check out dwarf rainbow fish such as forktail rainbows, threadfin rainbows, etc... They're really entertaining and are striking fish to have. They actually need the extra space as they are very active and the males can be a bit territorial and are in constant display to each other.
