|Shown: Nitrogen Compounds|
We aquarists spend a great deal of time trying to deal with this by abusing a form of Cycle - the Nitrogen Cycle. We draft vast armies of several molar units of mass into our filters in hopes that we might turn Ammonia ions into Nitrates... which we still have to remove, because they're not non-toxic... just less toxic. Go figure.
In nature, this is a proper cycle. We can almost mimic it in vitro, by adding tonnes of fresh-water plants. The plants absorb the nitrates and use them either to power their own chemistry, or release free, non-toxic nitrogen gas. There are some anaerobic bacteria that do similar things: such as the pseudomonads. Thing is, all these processes are either too slow to be properly useful, or occur in the absence of oxygen.
Could a tank with no need for water changes, only top ups, be possible? Maybe. Imagine a sump within the stand of the aquarium, with an extremely large surface area. Water is continuously removed from the master tank and pumped into the sump, which is designed in such a way as to minimize surface exchange and allow an anerobic environment for the relevant pseudomonad bacteria. Water is then pumped from the sump to the tank proper. Somewhere in the loop, a proper, aerobic canister filter is functioning to produce the nitrates as described above.
That's fun, but that's also relying on biology, and anyone who knows anything about science knows that biology is way, way slower than chemistry, which happens at roughly the speed of physics.
Can we chemically remove nitrates? Sure. All sorts of compounds absorb them - Zeolite, for example, is a mineral that's used rather exensively. Instead of a massive bacterial colony in the sump, we could bubble in Nitrogen Dioxide, producing nitrogen gas and water. This, of course, is an engineering nightmare. We'd need an overflow to deal with the water created, and a very powerful HVAC system on the sump to draw off the nitrogen gas, not to mention any excess nitrogen dioxide, which of course is highly toxic and stinks to high heaven. NO2 isn't expensive as reactants go, but it isn't cheap, and would need to be replaced.
|Shown: Aimin' to misbehave.|
Yeah, biologically seems like the way to go. Remember, my water changes take me about an hour to do - longer if I'm testing - and I do them once a week. I could spend up to ten full days working on creating this sump system for my master tank, which would save me pretty much the full time, since the betta tanks don't actually need their water changes as often as I give them. As long as I kept all the tanks running for five years, I come out ahead or break even on the investment of time creating the system.
Conventional wisdom says you can't eliminate water changes, but I never liked being conventional. See figure two.