Few things, 1 there are benefits to water changes, not much benefit to not doing them aside from stability if you have everything dosing right where you want it. But there are things you may not be able to remove from the water column that dilution via water change will. What those things are we don't know, toxins, contaminants, disolved organics.
But I don't change my water.
However I also rarely change our fw, only when I see the airation creating bibbles that don't pop. Which is absurd how far you can go.
We have a 55G goldfish tank with 9 very large fancies, I have gone months without changing it, the nitrates accorsing to api via dilution is somewhere in the 130 range, but everything is going fine. Is there a benefit to this? No, we need to supliment calcium because its not bring introduced by water changes, well, aside from less work... My wife feeds them super heavily, and some of them are 9 inches long mouth to tip of tail (big tails though) we have a 5 inch substrate on it to allow extra denitrification, and lots and lots of plants for nutruent export.
There are ways to go without water changes, there are situations where its beneficial to stop then for a time. My reef is doing astoundingly well, and I have to add nutrients because my nutrient export outcompetes my heavy feeding. But I imagine, it could look just as good if instead of playing mad scientist and locking in a bunch of dosing pumps to make up for it, it could look just as good replacing the effort of extra testing doing math and keeping schedules, for the effort of moving 40 gallons of water every week or 2.
On the otherhand, the traces and carbon are all cheaper than salt for me.
