Nitrate and water change calculator
Enter the current NO₃, how much it rises per week and how often and how much water you change. The calculator shows the steady level, a 12-week course and the change needed for your target.
after change – before change
To keep NO₃ below 20 mg/l before a change, change at least 50 % of the water every 7 days.
Finding the weekly rise
Test NO₃ right after a water change and again before the next one. The difference per week is the rise – from food, fish waste and nitrogen in fertilizer, minus what plants take up. In a densely planted tank it can be zero.
Why NO₃ levels off
Each change removes the same share of nitrate, so the more there is in the water, the more a change takes out. After a few weeks the level settles between the value after a change and before it. A 50 % change once a week keeps pre-change NO₃ at twice the weekly rise, a 25 % change at four times.
How much NO₃ is fine
Planted tanks usually sit at 10–20 mg/l, with full fertilizing 20–30 mg/l. The result is an estimate: it assumes a steady rise and tap water without nitrate. If your tap water contains some, subtract it from the target.
More tools
- Aquarium CO₂ calculator from KH and pHWork out the CO₂ level in your aquarium from KH and pH or from the pH drop against degassed water. With notes on when the formula fails.
- Aquarium volume and substrate calculatorWork out your aquarium volume from its dimensions, how much water is left after substrate and hardscape, and how many litres and bags of substrate a slope needs.
Guides
- Cycling a planted aquarium: the first weeks and when to add fishHow long cycling a planted tank takes, what to test, how often to change water with aquasoil and when it is safe to add shrimp and fish.
- Aquarium water changes: how often and how muchHow much water to change in a planted aquarium, how often, what to watch with temperature and chlorine and how to combine it with trimming and cleaning.
- Algae in a new aquarium: which are normal and what to doBrown diatoms, hair algae and green dust in a new planted aquarium. Why they appear, which go away on their own and what helps without chemicals.
- Fertilizing a planted aquarium: macros, micros and target levelsWhat aquarium plants need, guideline levels for NO₃, PO₄, K and Fe, the difference between EI and lean dosing and how to spot a deficiency.