Originally Posted by zof
While I agree that if you let nitrates get to 30 or 40 ppm to signal a water change you have already failed, its important we know how much and how fast waste is building up in the aquarium. While 50-70% water change might be a good catch all for a water change many times with a light stocking in the aquarium a 20-30% water change will accomplish the same as the 50% in a fully stocked tank. Sure the fish wouldn't mind the extra fresh water but do we really need to waste that much extra water for lighter stocking on the tank (unless of course you drain the water into a garden in which switching the extra water you would put in the garden with the tank water would make perfect sense). The testing we do is a tool to help gauge these things and should only be used to help estimate what is right for your aquarium but is not a trigger for those things.
I think we're really talking of two different things. First, there is the stuff that causes nitrates to rise (if unchecked), namely the visible "waste" from fish that become organics and should be handled by the bacteria and live plants. Second, there is the "crud" that you simply cannot measure, but we know it is there because there are fish.
Both are in proportion to the fish load, true, but they are not necessarily related. As an example, in my tanks with all my plants, the "waste" is easily being handled. I see nitrates < 5ppm and have for years. It doesn't matter how many fish, relatively speaking; the nitrates are minimal because of the plants, snails and bacteria. So if I took nitrates as an indicator of a water change, I would never do one, and my weekly water changes are really not impacting "waste" at all because I never remove it from the substrate. Of course, if I never did a water change, the nitrates might rise. But with all my plants, I'm not sure this might not take a while, by which time the "crud" issue would be out of control. And given the number of fish I have in my tanks, the "crud" is the real issue and it is completely undetectable, or at least until it is too late when the fish's response would alert me if I let it go that far.
The fact that my fish respond so enthusiastically to a water change when all the test parameters remain excellent is proof of this as far as I'm concerned. The invisible and undetectable "crud" is getting removed by 50% every week, and this has a significant impact on fish health.
It is true that with few fish the "crud" would be less too. Those planted tank folks advocating no water changes are quick to point out that it works if the fish load is "moderate" and the plants are heavy. I recall reading one author who said the "no water change" works fine provided the fish are "balanced" and the example he used was 6-7 neon tetra in a 55g tank. The plants could handle everything at this level of balance. Most of us keep more than half a dozen small fish in a 55g tank.