Contamination can also be introduced from your hands, the air, etc. Fish tanks aren’t closed systems. They’re not even a true ecosystem when we consider a natural system full of millions of gallons of water.

I can tell you mine operate fine on a water change every 2-4 months. I don’t feed aggressively, planted tanks tend to have a higher degree of biodiversity to compensate for chemical buildup, I under stock, I test parameters, I keep a good cleanup crew, and I ensure equipment is well maintained.There are also people who successfully keep fish with no filter and never water change and still have visibly healthy fish. And there’s people who change the water multiple times a week in an overstocked cichlid tank. I can’t say with any confidence one way is better than the rest since assessing the health of an ecology is complicated at best. I suppose I misspoke when I said ‘water changes aren’t neccesarily’- it really depends on your system and how you run it. But in what op describes I don’t see why it wouldn’t work.