The reason for water changes obviously requires explanation. But first, none of us can ever know our fish are "happy," and "thriving" is probably something most aquarists also cannot discern. But we can be confident that the fish will be closer to that, and without question healthier, if we understand the need for regular substantial partial water changes.I believe that if an aquariums water parameters are fine and the fish are happy and thriving then why do water changes? now I’m not saying just don’t do water changes but if you test your water and the tests come back saying that your parameters are fine then why change the water. i know not everyone is going to like me for this, but I have not done a water change on my 36 gallon in 2 months simply because all the fish are healthy and thriving, I have not lost a single fish due to bad water parameters. I do top off water every week which normally is like 2 gallons. So as long as you test the water on a strict schedule and the water parameters are okay then I don’t think you have to change water on a weekly basis.
The water in an aquarium degenerates fairly rapidly if fish are present. No filter can deal with this, no matter how many or how large. Live plants can help but this is very limited and certainly no where near the level that would avoid water changes.
Freshwater fish are continually taking in water through every cell on their body. This is called osmosis, and it is behind osmoregulation in fish. This water passes through the kidneys via the bloodstream and is altered. The urine that fish excrete, which is considerable in relation to their physical size, is not urine like that which occurs in terrestrial animals but it is "dead" water. In a very short space of time, the fish are swimming around not in fresh water but in urine.
But there is more. Fish communicate visually but also chemically. They release pheromones and allomones, and the former are read by others in their species and the latter are read by other species. As just one example of how considerable the effect of this is...an aggressive fish will release allomones/pheromones that the other fish read as aggression aimed at them, and these will stress out the fish as much and often even more than physical aggression.
We cannot see any of this. There are no tests for any of the above. It just occurs and we must recognize it, and be prepared to deal with it. And no filtration we use can handle any of it. Only water changes.
As for tests we can do...by the time these indicate the need for a water change, it is too late. The fish have already been impacted and it may or may not be reversible. Nitrate should always be at the same level whenever it is tested; if it rises before the next water change, there is a serious problem. Either the water changes are not substantial to handle it, or there are too many fish, or they are being way overfed.
A regular (meaning one every week) substantial (meaning 50-70% of the tank volume) water change should be standard. Fish will never be harmed by water changes (assuming the parameters between tank water and fresh water are relatively similar) and it is the single most important factor in healthy fish. And that is what really makes fish thrive.