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Water change calculator
Work out how many litres you change and how far nitrate drops, or how much you need to change to reach a target value.
How it is calculated
Litres = water volume × share. With 160 litres and 30 % that is 48 litres.
Nitrate afterwards = nitrate before × (1 − share) + tap water nitrate × share. Example: 40 mg/L in the tank, 10 mg/L in tap water, 30 % change: 40 × 0.7 + 10 × 0.3 = 31 mg/L. The old rule “30 % change, 30 % less nitrate” only holds when the new water contains no nitrate.
The other way round: share = (before − target) ÷ (before − tap water). To go from 40 to 20 mg/L with 10 mg/L in the tap water you need (40 − 20) ÷ (40 − 10) = 67 %. Two changes of 42 % each achieve the same more gently.
How much and how often?
- Normally stocked tank: 20 to 30 % per week.
- Heavily planted, few fish: 20 % every one to two weeks is often enough.
- Nitrate guideline: below 25 mg/L for most tanks, below 20 mg/L for shrimp and sensitive species. The app checks this against the animals in your tank.
- Fresh water should be close to tank temperature; use a water conditioner with tap water.
In the app: log a water change with one tap
In Aqualizer’s quick menu you enter the water change as a percentage. The reminder restarts, the log records it and the nitrate history shows whether your routine is enough.
- Reminder on your own schedule
- Nitrate history with trend
- Target range matched to your animals
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Join the beta on Android
Water change FAQ
How much water should I change?
For most tanks 20 to 30 % per week. That keeps nitrate and other substances low without stressing the animals with big jumps in temperature and hardness.
How far does nitrate drop with a water change?
By the share you change, if the new water contains no nitrate. If your tap water brings nitrate itself, it drops less: with 40 mg/L in the tank, 10 mg/L in the tap water and a 30 % change, to 31 mg/L.
Is a large water change bad?
Not in principle; with nitrite or ammonia it is the right thing to do. More than 50 % at once, however, changes temperature, pH and hardness abruptly. If there is no rush, two smaller changes a few days apart are gentler.
Why can’t I get nitrate any lower?
Because your tap water contains nitrate itself (up to 50 mg/L is allowed in the EU). Water changes cannot get below that value. Mixing in RO water, more fast-growing plants or less food help.
Where do I find the nitrate value of my tap water?
In your water supplier’s drinking water analysis, usually on their website by postcode. Alternatively, measure it at the tap with a nitrate test.