How does alkalis pH affect fish?

Different fish require different water conditions. For a fish that requires a low or acidic water (such as the beautiful Amazonian 'discus') or a fish that requires a high or alkaline water (such as the vivid 'electric blues' or 'yellows' from the Rift Valley in Africa)to be kept in water of the wrong pH would simply mean that the fish will probably be unable to breed, will never thrive and may have a much shortened life span.