John, it’s my impression that people have been importing fish from other continents for many decades. Also, well water vs river water can make a big difference locally.
It sounds like you are suggesting we all keep fish from local rivers and lakes (which I know you don’t mean, but is the best way to assure they are adapted to your water). Most of us don’t want to do that — we want to raise tropical fish. Which means imported.
I don’t know (not an expert) but I suspect fish don’t adapt to different water conditions in just a few generations. I believe evolutionary changes are much more complex and much slower than that.
Which is why I bought an RO filter, even though the city water here is fairly soft (150 TDS).
No, I'm not suggesting that, although it would remove one big source of potential problems. But I've always been a fan of keeping fish that have evolved in and are adapted to water that is similar to the water that is readily available to me. That doesn't mean they have to come from a local river; the same conditions can exist in many places; like I said, my tap/well water is pretty much what many African cichlid fans strive to create artificially for their fish. And of course, tapwater in the city
could be significantly different than water out of a well. If that's the case, it's a possible complication, but doesn't change anything regarding the suitability of one over the other for specific types of fish.
And I agree that fish aren't evolving in a very brief generations in captivity. But individual specimens that come from soft/acid water and are then thrust into hard/alkaline water must either adapt to the sudden change, or not. Some specimens won't make it. Whether the transition occurs at the importer level or the home aquarium level may not make any difference to the percentage that die in the process, but it can make a big difference to the aquarist who buys a bunch of fish and then loses 25% of them as opposed to the aquarist who buys the same number but from a group that have already been culled by the shock of a huge change in parameters. It would be like me taking a netful of one of my fish species that are currently living at 55F in my basement...and dumping them into a tank of 80F water. That would be very stressful for them, and I suspect many would perish. They will be living in that temp without problems next July...but the change will have been gradual, not abrupt.
The degree of adaptability is bound to vary from species to species. A species found in a small area in nature, with stable conditions that don't change much all year and which don't vary across their entire native range, is perhaps much more unforgiving of change. Another species that is found over a vast geographic range, encompassing many different water types, perhaps ranging even into brackish or marine environments, perhaps experiencing large seasonal changes in temperature or other parameters...well, those are likely going to be tough, adaptable fish that can do well almost anywhere.
The grass is always greener
over there...
We have aquarists with soft/acidic tap water spending time and money adding minerals to it so that they can keep rift lake cichlids...while at the same time other aquarists with hard/alkaline water are investing time, energy and money in RO units to make their water good for delicate Amazonian species. We have many aquarists living in the cold north running electrical heating devices to keep their tanks at tropical temperatures in cold conditions...while at the same time we have aquarists in the tropics buying chillers because they want fish that can't take the heat. We have aquarists in the centre of North America buying giant bags of salt so that they can create "marine" water hundreds or thousands of miles from any ocean...while at the same time we have people living at the seaside, sometimes practically on the beach, who use tap water made by desalination plants to fill their fresh-water aquariums. The common factor here is that all of those people are fighting nature and they...or more accurately, their fish...are at the mercy of technology.
People are funny creatures.
