It seems to me that our aquariums are a veritable cesspool of organic waste (nitrates) compared to the natural environments our fish our found in. I have never seen a fish react adversely to water change between 50-100% of the water. Obviously, if you change 100% of the water you would need to acclimate the fish to the water just as you would if you purchased a new fish. The newer water is so much freer of this waste it couldn't possibly have more of an adverse effect on them than keeping them in the same level or a slightly reduced level (10-25% wc) of nitrates.
Actually, our aquariums should be an example of a balanced eco-system which means that there should not be an ultra high concentration of "end by-products" such as nitrates. Since we are dealing with a closed system, that isn't always possible which means we need to take more action to keep the levels from becoming toxic. By doing smaller, more frequent water changes, the level of nitrates should remain low in a balanced tank. A good aquarist (IMO) will be mindful of not overcrowding their tanks. Overcrowding a tank purposely is really a questionable method of fish keeping and the cause of a lot of problems. While overcrowding is a typical method of keeping African Cichlids, there are other methods of keeping smaller quantities of these fish safely.
Re: 100% water changes, constantly needing to re-acclimate your fish after doing this will eventually take it's toll on the fish. Fish prefer consistency over radical change( even if the change is to better water quality.) Gradual change to the bad is more tolerable than rapid exposure to bad conditions. (Same goes for good water as well.) It's the reason many fish that are brought into pet stores for trade ins don't always survive. They don't handle the change from their poor environments from the home to the better environment in the store because even with acclimation, the time frame for doing this is usually longer than 1 hour ( the timeframe some use as acclimation time.) Obviously there are exceptions but this is more about the rule than the exception.
Re your comment" I have never seen a fish react adversely to water change between 50-100% of the water." is a pretty incredible thing to me or maybe you have not been exposed to that large a variety of fish. As an importer, I had to constantly be adjusting how much water was being changed in tanks as some of the fish could handle more rapid changes than others. I HAVE seen fish react adversely to water changes between 50%-100% so let's call that one a draw.
Re your comment: "The newer water is so much freer of this waste it couldn't possibly have more of an adverse effect on them than keeping them in the same level or a slightly reduced level (10-25% wc) of nitrates." If we were to take a nitrate level of say, 10 PPM and you change 10%, it will reduce it to 9 PPM, not a major change and if you do 25%, you go from 10 down to 7.5 PPM which is still not a major change as all of these levels are happening within the acceptable levels of a healthy tank. Now say you have a level of 100PPM and you do 10%, it brings it to 90 PPM which is a decent change but if you do 25%, you go down to 75 PPM and THAT is a major change for a fish and major changes are not good for fish.
Hope this explains it
