Water change & nitrate calculator

Find the water change that takes your nitrate to a target, then see how nitrate behaves over weeks of regular changes, including where it levels off.

Inputs

Total water in the system, including the sump.

From your test kit. Most hobby kits read mg/L as nitrate (NO₃).

Pick what suits your livestock; reef tanks usually aim far lower than fish-only tanks.

RO/DI water is 0. Test your tap water: any nitrate in it limits how low water changes can go.

The routine change used for the chart.

Test just after a change and again a week later with no change; the difference is the rise. Use 0 to see pure dilution.

Water to change

27.5gal

50% of your 55 gal tank takes nitrate from 40 to 20 ppm.

Changing 25% weekly keeps nitrate between 15 and 20 ppm.

Change needed
50%
Nitrate after
20.0 ppm
To reach target at 25% weekly
5 changes (by day 28)
Schedule settles at
15.0–20.0 ppm
Nitrate before (solid) and after (dashed) each regular water change
010203040010203040Time (days)Nitrate (ppm)Reaches target: 20–26 ppm

A single 50% water change (27.5 gal) takes nitrate from 40 to 20 ppm. On a regular 25% weekly schedule the tank adding 5.0 ppm between changes settles between 15.0 and 20.0 ppm, reaching the target after 5 changes (the first is today). The chart plots the level just before each change (solid line) and just after it (dashed line).

How it works

A water change is a mixing problem. When you replace a fraction of the tank with new water, the nitrate left is a volume-weighted average of the old and new water:

after = before × (1 − f) + new × f
f (one change to a target) = (now − target) ÷ (now − new)
settles at (just before a change) = new + rise ÷ f
  • f is the fraction of the tank replaced, new is nitrate in the new water (0 for RO/DI), and rise is the nitrate the tank adds between changes.
  • The first line is the same dilution equation chemists use (C₁V₁ = C₂V₂); with nitrate-free new water it reduces to “a 50% change halves nitrate”.
  • The chart repeats the change on your regular schedule. The solid line is nitrate just before each change and the dashed line is just after, so the band between them is what your fish live in.
  • Settles at is where nitrate added between changes equals nitrate removed by the change. A tank adding 5 ppm a week with a 10% weekly change levels off at 50 ppm before each change.

Worked example

A 55 gallon tank at 40 ppm nitrate, tap water at 0 ppm, target 20 ppm. The tank adds about 5 ppm a week.

  • One change: (40 − 20) ÷ 40 = 50%, which is 27.5 gallons.
  • Keep it there with 25% weekly changes: the tank settles between 15 and 20 ppm.
  • Starting from 40 ppm, that schedule gets below 20 ppm right after the fifth change, on day 28.

Tips from the fish room

  • Test the new water. One nitrate reading of your tap can change the plan. The calculator’s “new water” box is there for it.
  • Measure your own rise. Test after a change and again a week later with no change. The difference is the number for the “adds per week” box, and it tells you more than any stocking chart.
  • Regular beats heroic. A steady 25% weekly change holds nitrate in a narrow band. A big change once a month swings it up and down.
  • Match the new water. Temperature, and for reef tanks salinity, should match the tank before it goes in.

FAQ

How much water do I change to lower nitrate?

Divide the drop you want by how far the tank is above your new water. With nitrate-free new water, going from 40 to 20 ppm is (40 − 20) ÷ 40 = 50%. For 80 to 20 ppm it is 75%. If your tap water holds nitrate, the percentage goes up, because the new water brings some back.

Does a 50% water change really halve nitrate?

Yes, if the new water has none. Half the nitrate leaves with the old water and the new water adds nothing. Two 50% changes leave a quarter, not none, because each change only removes half of what is left.

Why is my nitrate stuck even though I do water changes?

Two things set the floor. Nitrate in your new water can't be diluted below its own level, and the tank keeps making more between changes. On a regular schedule nitrate settles at the new-water level plus the weekly rise divided by the fraction changed. A tank adding 10 ppm a week with 10% changes settles 100 ppm above the new water, which is why a bigger or more frequent change works better than waiting.

Is this nitrate as NO₃ or as nitrate-nitrogen?

The calculator works in ppm (mg/L) whatever the form, but the target has to use the same form as your test. Most hobby kits read NO₃. Some, especially reef and lab-style kits, read nitrate-nitrogen (NO₃-N); multiply an NO₃-N reading by 4.43 to get NO₃.

How big a water change is too big?

Size is less of an issue than a mismatch. A large change is fine when the new water matches the tank's temperature, pH and salinity, and riskier when it doesn't. The calculator flags anything over half the tank so you can decide to split it over a few days.

Sources

  1. Water Change Parameter Calculator, Reef Exclusive. Steady state is the change per period divided by the water change fraction; 5 ppm a week with a 10% change settles at 50 ppm.
  2. Water Change Calculator, AquariumCalc. Change needed = ((current − target) ÷ current) × 100; 40 to 20 ppm is a 50% change.
  3. Concentration of solutions (Introduction to General Chemistry), LibreTexts. The dilution relation C₁V₁ = C₂V₂ behind every water change; the worked HCl example is used as a test of the mixing equation.

Formula and sources last checked September 30, 2026. How we test formulas.