A water change removes the same share of nitrate as the share of water you change: 10% of the water takes out about 10% of the nitrate. Enter your volume, change size and current nitrate to see the litres and the result. Add how fast nitrate rises in a week to see where your routine settles.
Fill in your volume and today’s nitrate to see what each change does.
The maths
Nitrate after a change = current × (1 − fraction changed) + new water nitrate × fraction changed. With RODI water, new water nitrate is 0. If your tank adds a steady amount between changes, nitrate settles where each change removes exactly what was added since the last one.
| Routine | Litres each time | After the next change | Settles between |
|---|---|---|---|
| 10% weekly | 20 L | 18 mg/L | 9–10 mg/L |
| 20% every 2 weeks | 40 L | 16 mg/L | 8–10 mg/L |
| 20% weekly | 40 L | 16 mg/L | 4–5 mg/L |
Finding your weekly rise
Test nitrate just after a water change and again a week later, before the next one. The difference is roughly your weekly rise. Your Companion test log makes this easy to read from the chart.
What it can’t tell you
Skimming, bacteria and algae remove nitrate too, so your real levels sit below the settled range shown. The calculator is for comparing routines, not predicting an exact reading. For high nitrate, fixing the source usually matters more than the size of the change; see high nitrate.
Common questions
How many water changes to halve nitrate?
About 7 changes of 10%, or 4 of 20%, if nothing else changes.
Does it work for phosphate too?
The dilution maths is the same. Enter phosphate figures instead and read the units as mg/L phosphate.
Should new saltwater contain nitrate?
No. Mixed with RODI water, it should test at zero.
Sources and limits
Last checked 12 October 2026. The calculator uses simple dilution maths and ignores other export, so real levels sit lower than its settled range.
Keep going
Water change calculator · Free Reef Companion · All calculators