Water change calculator
How many litres to drain — and what the change actually does to dissolved waste.
Drain and refill
45 L≈ 11.9 US gal
Dissolved waste left after the change: about 75%
A water change is dilution, and dilution has arithmetic that surprises people. Change 25% and you remove 25% of the nitrate — not all of it, and not most of it. Do that every week against a tank that produces a steady load and the nitrate does not fall to zero; it settles at a plateau roughly four times the weekly production, which is why a tank on 25% weekly changes sits at a stable 20 ppm forever rather than creeping down. Want it lower? A 50% weekly change halves the plateau. Two 25% changes in a week barely beat one 50% change, because the second one is diluting water you already diluted. The same maths runs backwards for anything you add: dose a fertiliser weekly and change half the water the next day, and you threw away half of it. Beyond the numbers there are two rules worth more than any schedule. Match the temperature — a swing of more than a degree or two is a real stress event, and cold refill water is a classic trigger for ich a week later. And dose dechlorinator for the whole tank volume, not the bucket, unless the bottle explicitly says otherwise: chloramine breaks into chlorine and ammonia, and the ammonia half needs the whole system's worth of binder to catch it.
Common questions
- How much water should I change weekly?
- For a typical stocked community tank, 25–50% weekly is a solid habit. More stock, more food or visible algae push you toward the higher end; a lightly stocked, heavily planted tank can run on less.
- Does a bigger water change remove proportionally more nitrate?
- Yes — dilution is linear. A 50% change halves nitrate; two consecutive 25% changes remove about 44%, slightly less than one 50% change, because the second change dilutes already-diluted water.
- Is a big monthly change as good as small weekly ones?
- For nitrate, a single 50% monthly change leaves a much higher average than 12.5% weekly, even though the volumes match — the tank spends most of the month at the top of its sawtooth. For the fish it is worse still: large infrequent changes swing pH, hardness and temperature all at once. Little and often wins on both counts, and the tool shows you the plateau each schedule settles at.