I never did a water change.
I just tested the water with an Aquarium Pharmaceuticals Freshwater Master Test Kit which I purchased today (probably the third I've purchased, just wanted new reagents).
Fish consume food, and fish and unconsumed food produce ammonia, which bacteria convert to nitrite, which bacteria convert to nitrate, which you must remove from the tank via regular partial water changes. Because I haven't done this, my nitrate should be through the roof, and my fish should be dead. But they're quite perky and otherwise healthy looking, and a lot bigger than they were a year ago.
- Plants consume nitrate.
- I have always been very careful to only clean the front glass - nothing else in the tank has been touched.
- There is direct sunlight on one end of the tank for part of the day.
- So there is a very thin but complete layer of algae on that end of the tank.
- The tank has two bio wheels, which I believe are great.
- I never put anything but food and water in the tank.
- I don't overfeed.
- My tank is not overstocked.
I do not recommend stopping your water changes, because I don't know why my fish aren't dead. But I do recommend that other than feeding, water changes, and cleaning the absolute minimum glass for viewing, you leave your tank alone. And I'm only talking about fresh water.
Oh, I really don't know about vacuuming gravel. I haven't been doing it. I think I remember hearing scary stuff about anaerobic bacteria. But I just found:
A denitrator uses anaerobic bacteria to reduce nitrate to nitrite and then to nitrogen gas. It works by flowing water slowly through a filter with a lot of surface area for bacteria to colonize. Aerobic bacteria near the front of the filter use up the oxygen in the water, creating an environment for anaerobic bacteria to do their work reducing nitrate.Which sounds kind of familiar.
Deep sand bed in marine systems help reduce nitrate by providing a place for anaerobic bacteria to colonize.
The filter is a Marineland Penguine 330 bio-wheel power filter. The filter cartridges haven't been replaced in the five years I've had them - I slit them open and removed the carbon, and rinse them out occasionally, but I don't think the rinsing, or even the filter cartridges at all, are actually useful.
I would guess it's a good idea to do some water changing at the least to reduce the concentration of things added with top-off water that are not removed during evaporation.