water

Water change calculator

See what a water change of a given size actually does to a measured parameter — and how many changes it takes to reach a target.

Tank water volume (L)

Change size (%)

Current reading (mg/L)

Same parameter in replacement water (mg/L)

Tap water nitrate is often not zero.

Target (mg/L)

5 changes to reach 20 mg/L

Each change removes 30 L. After one change: 45 mg/L. After three: 27.15 mg/L.

Working — every step, so you can check it

Formula

After one change: C₁ = C₀ × (1 − f) + Cₛ × f
where f = change fraction, C₀ = current concentration, Cₛ = concentration in the replacement water
After n changes: Cₙ = (C₀ − Cₛ) × (1 − f)ⁿ + Cₛ
Changes needed to reach target T: n = ln((T − Cₛ) ÷ (C₀ − Cₛ)) ÷ ln(1 − f)

An estimate whose assumptions are invisible is indistinguishable from a guess.

The tank is perfectly mixed, so the removed water carries the current concentration.
Nothing is produced or consumed during the change.
The replacement water's concentration is constant.

Where this calculation stops being reliable. Read these before acting on a result.

Dilution is the only mechanism modelled. Ongoing production from fish and food means the level starts rising again immediately.
A target below the concentration of your replacement water is unreachable by water changes at any size.
For a tank that has been chronically high for months, reaching the target in one change is not desirable even though the arithmetic allows it. Spread it over several days.