There are all kinds of "beliefs" about what fish will and won't adapt to. Here are my experiencse. I'm on a well, no chlorine. My pH is typically 8.0 to 8.5, sometimes as high as 8.8. The dGH runs 25 to 40, and dKH is 9 to 15. I'm sure there have been times when those numbers were either lower or higher, but these numbers are the ranges they've fallen into when I've tested. Anyways, the simple translation of all this is that my fish live in that legendary substance known as "liquid rock". Seriously. My water is actually harder than more than a few of the Rift Valley lakes. It is still fine for breeding Rift Valley cichlids, Central American cichlids and livebearers with no issues whatsoever. All three of these groups are considered compatible with higher pH and harder water anyways. Here's the rest of what I've kept and bred beyond these three groups.
Since I've been on this well, I've had a couple of species of West African cichlids, angelfish, several species of badids, several species each of rasboras and danios, white cloud mountain minnows, bettas, bristlenose plecos and a few other species spawn, all typically considered soft water, low pH species. I've maintained several species of cories, several species of barbs, a dozen or so species of tetras, other South American cichlids, a number of pleco species, some other SA catfish, leopard ctenopomas, a couple of species of loach and other species that I don't remember right now without any hassle. All the fish listed in this paragraph lived or are living healthy, active lives, and I've seen no reduction in them living a normal lifespan.
A couple of caveats here. First, only the badids, one of the West African cichlids, a couple species of the cories, and the miscellaneous plecos were wild caught. The rest of them were captive bred AFAIK. Second, I have no illusions that none of the tetras, cories, miscellaneous pimelodids, loaches, etc. are going to breed. Unless I condition the water it just isn't going to happen.
So, IME, most fish can adapt to a far wider range of conditions than what is generally considered "acceptable", and can and will be healthy. Now this is where some people will start calling me a "heretic", lol. And that's fine. My experiences tell a much different story than conventional wisdom, and after a little deeper thought it actually makes sense that fish can adapt to a wider range of water conditions when you think about it.
WYite