Hi all,
I appreciate that we are all going to differ in views, but there really are scientific reasons why fish will be stressed in the "wrong" water.
If we stick with the alkaline end of the spectrum, we have a really good example of adaptation to "extreme" conditions, it is the sea and marine fish.
The sea is an immense volume of water and, particularly in tropical seas, extremely stable. Neither temperature nor salinity vary, and with very few exceptions fish have very restricted tolerances of changes in salinity or temperature. This is why marine reef keepers have chillers, hygrometers etc.
These are also very low nutrient conditions where even trace levels of nitrate (NO3) or phosphate (PO4) can trigger algal blooms, coral death etc.
If we look for a fresh water analogy, we actually have one - Lake Tanganyika, ancient, huge volume of water, stable temperature, highly buffered, nutrient poor etc and full of fish with some very specialised adaptations. When people started keeping Tangs they really struggled because we didn't realise just how odd the water was. We don't struggle so much now because we understand more about the water and it is relatively easy to add compounds to water to raise the alkalinity, GH and KH.
What you won't find even now is many Tangs that are cheap to buy or can be kept in ordinary water, some species are more tractable - Neolamprologus pulcher for example, but they still need hard alkaline water, even after many generations of artificial breeding and selection. The age and stability of their lake has meant that the genes that would have allowed them to thrive in different conditions have gone, in the same way that the Blind Cave Tetra has lost the genes to produce pigment or eyes.
This situation isn't true for the vast majority of tropical fish we keep. They come from rivers and non-rift lakes which are transient in evolutionary terms and produce species which can and do change, adapt and mutate over time through the action of natural selection on the genes they pass from generation to generation. It is this combination of natural selection and artificial selection that has now bred Cardinal and Neon Tetras etc. that can survive in water which is much harder and more alkaline than their natural habitat.
If we want to find fish that are a real challenge to keep we would need to find a tropical region where stable conditions over millions of years have allowed specialised fish to fill a range of evolutionary niches, ideally in very nutrient poor water with extremely low TDS (You need to remember that adding salts to water is easy, but taking them away fairly difficult).
I've just described the black and clear water rivers of the tropical rainforest zone, and their specialized fish - Panaque, Panaquolus, Cochliodon the worlds only wood eating fish, the huge speciation of the Hypancistrus's, the plecs in general, the Tetras, Geophagine Cichlids etc.
Plec keepers are lucky in that relatively few of their fish come from black waters, but many of the fish that do, Chocolate Gourami, cichlids like Dicrossus, Satanoperca, Apistogramma diplotaenia, Heckel Discus etc. are some of the most difficult fish to keep. They demand very clean water because they have little resistance to diseases (their acid water bathes them in constant very dilute disinfectant) or parasites, and they often need careful feeding because they come from water which is poor in food, and where any food item will be eaten, even after the fish is stuffed with unsuitable food.
So why can we get away with keeping Bristlenoses in akaline water? It is because they have been bred over generations (by natural and artificial selection) for hardiness, because they originally came from rivers where conditions, temperature, water quality were neither extreme nor very stable and because they had some genetic variability for natural and artificial selection to work on (probably because they show "hybrid vigour" following the mixing of genes from related species).
This is also why fish like Apistogramma cacatuoides & A. agassizii are reasonably common in the hobby, and many other fantastic Apistogramma species aren't. The black water fish will never be commonly kept, they are too difficult to keep and even more difficult to breed.
cheer Darrel
