Hi all,
also always thought that pure water and fish didn't mix, if a fish is kept in water of the wrong mineralisation i.e soft water fish in very hard water that the fish can get weakened by it's body trying to regulate it's salt content with water it has not adapted to live in. If the water is totally devoid of minerals even soft water fish may not be happy. "
I think that the problems with very soft water depend upon the fish, but it is definitely best to keep hard-water fish in hard water etc., but I'm not sure how much of this is to do with osmo-regulation. You can think of a fish like a Brown Trout (
Salmo trutta) which is happy in a full range of salinities from salt to fresh, and a range of carbonate hardness levels from 0 dKH through to water that is fully saturated with calcium carbonate, the critical requirement for trout in any water is a high level of dissolved oxygen. Acididfication has killed many trout fisheries, but this is mainly to do with the toxic effects of soluble aluminium ions etc on invertebrates and fish eggs rather than the acid water itself.
As a general rule carbonate rich water support a lot more in the way of invertebrate biodiversity, so grow a bigger range of larger fish. If you take a large lake like Lake Tanganyika it is almost as stable as the sea, it has an immense volume of very highly buffered warm water and has existed as a lake for a huge amount of time. The fish have evolved in a stable environment and have very little tolerance of any other condition, they have also evolved to fill very small specialised niches.
Soft water "Black water" fish will usually have more tolerance of changing conditions, but the extremely acidic nature of their environment drastically reduces bacterial activity, and they may be prone to bacterial infections etc when kept in harder water, there may also be problems with egg shell hardening etc. A further potential problem is that they also tend to be opportunistic feeders, eating anything and everything, often leading to problems like bloat. As a general rule acidic water hold much less biomass than more base rich ones, although exceptions may occur in exceptional conditions (the igarapes and flooded forest of the Amazon basin for example).
cheers Darrel