We can use [NO3] (nitrate concentration) as an indicator of organics because NH3 is a product of fish metabolism, so in a cycled tank NO3 is associated with NH3 and so NO3 is associated with fish wastes.
If we assume that at least the majority of organics come from fish, or at least that all organics are produced with a concomitant release of ammonia, then NO3 can be used as a proxy measure of the organic pollutant load.
<Fish also excrete heavy metals, I forgot about that in the earlier post.>
The same approach can be applied to NO3 measurements as I showed for our imaginary pollutant. Simply consider that fish produce 'x' amount of ammonia in a cylcled tank in a week. If you're doing 50% WC's weekly, your NO3 levels should level out at x/0.5 or twice the concentration produced weekly after a set number of weeks.
Yes, monitoring NO3 is the most quantitative way a hobbyist has to know whether enough water is being changed. Choose your benchmark, be it 10, 20 or 40ppm. However, there are a couple things to think of, first many folks won't want to buy test kits, second if there are plants in the tank you lose the association between pollution and NO3.
If based on your stocking load (amount of fish vs. tank size) and feeding routine and 25% change is sufficient to maintain NO3 at a stable level below your chosen benchmark, then 25% weekly is all that's necessary in your tank. Like got_nailed said, it really depends on what fish, what tank, how often you feed, etc. A lot of variables.
But no, toxin levels will not continue to rise after the tank reaches its equilibrium. You can measure it using NO3 and an estimate of weekly change percent.
Your equilibrium [NO3] = weekly input of NO3/percent water changed per week
Someone handy at math could work out the Taylor series for it and figure out the amount of time necessary too, I just used Excel and auto-filled
Eg. if after a week your NO3 increases by 5ppm and you do a 25% water change weekly:
[NO3]equilibrium=5ppm/0.25=5ppm x 4=20ppm
Neither NO3 nor associated toxins will increase in concentration beyond this level.
I know it can be tough to get your head around, but maybe this will help.
Let's go back to the earlier example of 100 units of an imaginary pollutant per week with 50% changes. But now lets follow what happens to the pollutant from the first week:
week 1: 100 -> remove half -> 50
week 2: 50 -> remove half -> 25
.
.
.
week 8: 0.75 -> remove half -> 0.38
Remember that the equilibrium concentration is about 200 units, so by now the first week's contribution to this is minute, negligible even. No, it will never be completely eliminated, but it will become infintessimally small.
) I have a stingray, so I change 50% 2-3 times a week. I use dechlor in the tank and try to match the temp as close as possible. I have never had any problems with my fish due to tap water. Speaking of.... I'm off to do a water change right now!
