Water change calculator
What a water change does to nitrate, phosphate, alkalinity or anything else you test, and where a weekly change makes a slow build-up level out.
Formulas and sources last checked . How we check
Calculator
Result
After 1 change18.00 ppm
- Old water replaced
- 10.0 per cent
- Same total, changed slowly and continuously
- 9.5 per cent
- Moved towards the new water by
- 2.00 ppm
How much does a water change lower nitrate?
A change moves the reading part of the way towards whatever the new saltwater contains. A 10 per cent change closes 10 per cent of the gap. Each further change closes the same share of a smaller gap, so two 25 per cent changes replace 43.75 per cent of the old water, not 50. Changing water slowly and continuously replaces less again, because some of the water you take out is new water. If something builds up at a steady rate, a weekly change stops it climbing at the point where each change removes exactly what the week added.
after n changes = new water + (now - new water) x (1 - share)^ncontinuous: share replaced = 1 - e^(-volume changed / system volume)levels out at = new water + weekly rise / share changed
Worked example: nitrate at 20 ppm
- Nitrate is 20 ppm and new saltwater has none. One 10 per cent change: 20 x 0.9 = 18 ppm.
- Four weekly 10 per cent changes, with nothing else going on: 20 x 0.9^4 = 13.1 ppm.
- If feeding adds 2 ppm a week, 10 per cent weekly changes level nitrate out at 2 / 0.1 = 20 ppm just before each change. To hold 10 ppm you need 20 per cent a week, or less food, or another export.
Do you need water changes at all?
| Topic | What sources say | Our reading |
|---|---|---|
| Regular water changes |
| Water changes are the simplest export and reset for most beginners. Skipping them works only with a method built for it and regular lab (ICP) tests. |
What are the limits?
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