50%
Change 5.0 US gallons to bring 40 ppm down to 20 ppm.
A water change removes a percentage of the nitrate in the water, not a fixed amount. If nitrate is 40 ppm and you change 25%, you end at 30 ppm. The tool below solves it the other way round: you give it the current reading and the target, and it tells you the percentage that gets you there in one change.
How to use the result
- Above 50% in a single change, split it: 30% now, 30% in a few hours. Two moderate changes are gentler on livestock than one drastic one.
- Match temperature and dechlorinate for the full tank volume, not just the replaced volume.
- Log the pre-change nitrate and the volume in your logbook — three weeks of that pair tells you whether your interval is right.
Why nitrate is the right anchor
Ammonia and nitrite should be zero in a cycled tank, so they are not useful for sizing routine changes. Nitrate accumulates predictably, which makes it the one number you can schedule against — and the one number worth logging every week.
