In my 16 gallon, I used to have a lot of plants. Now I'm fairly sure I have more plants than water - it is an absolute jungle. I have java fern, red ludwigia, elodea, and a couple of strands of hornwort. Oh, and a bunch of java moss. It may not sound like much, but it has all grown to the point where my tank is almost entirely green. It doesn't bother my fish, as my cories are bottom dwellers anyway and the rest are so tiny they fit between all the plants. My shrimp love it.
Anyway, the upshot of this is that I haven't cleaned my tank in two weeks (I know, bad!) but my ammonia and nitrite are still at zero, and my nitrate has not gone above 5, which is what it measured after my last water change. I'm going to do a bit of siphoning to get the shrimp poo off the sand, but only because it looks messy. This seems to confirm my theory that, despite not being an NPT (although there is enough crud mixed in with the gravel to *be* dirt), my tank is functioning by itself to the point that I only really need to top up for evaporation. I will keep testing it, of course, and the moment nitrates do anything I'll do a change.
I'll post a photo later. :)