well mostly it can be due to keeping imbalanced taks. I agree water changes are a vital part of fishkeeping. But in general the goal of fishkeeping is to duplicate nature within the confines of an aquarium. If a situation is created where there is no toxic buildup up of anything. and all waste is being recycled... Then what would be the need for water changes? The best thing we can remember if we love this is That There is no waste in nature. A healthy habitat will renew itself indefinitely unless acted upon. When we have to put so much work into maintaining an aquarium we are either
A. stretching the parameters of the fish we are keeping. Or not investing the time or resources to try to duplicate their environment. Not a bad thing if you're going the extra mile and doing water changes
. Just saying, In the wild fish keep themselves.