Originally posted by rus
tank raising does not change a fishes water requirements. fish do not adapt to water chemistry because they are born into them. evolution has molded the fish to the water chemisty in it's natural habitat and raising it in a tank is not going to change that. sure fish will live in water that is not like the waters they are found in and sometimes nowhere close to the waters they are found in, but they should not be kept in these waters. most fish do not thrive in water greatly unlike that of their natural habitat.
I'd like to offer a slightly different perspective.
Evolution indeed moulded the fish to its wild conditions, and that same evolution will also mould the fish to new conditions - such as those prevailing in aquaria.
Take Angelfish for example. Years ago they had a reputation like Discus - demanding about water quality and parameters. Now, you can breed them in chalky liquid rock; I've seen it done. Natural selection will consistently select those individuals that are more tolerant, because the picky ones end up belly up.
Some species seem more maleable than others - Rams for example seem still to be very picky.
According to Baensch vol. 2, guppies are another case in point. If you get wild ones, you need to keep them in water like the water they came from - this is very variable, but each population is adapted to its particular water conditions. Domestic guppies, OTOH, will adapt to a wide range of water conditions, apart from the very soft and/or very acid.
I sometimes wonder if poorly doing specimens are on occasion put down to the "wrong" water when in fact the fish is in poor condition because the DOC level of the water is very high, the nitrate is through the roof, and it's being picked on by the RTBS.