Water change calculator
How much water to remove for a given percentage, and how repeated water changes dilute nitrate or any other dissolved substance.
The math
Volume to replace = tank volume × percent ÷ 100.
If you replace a fraction p of the water with water at level Cr, the new level is C₁ = Cr + (C₀ − Cr) × (1 − p). After n changes (with nothing added in between): Cn = Cr + (C₀ − Cr) × (1 − p)n. The number of changes needed to reach a target Ct is n = ln((Ct − Cr) ÷ (C₀ − Cr)) ÷ ln(1 − p).
Worked example
Nitrate at 40 ppm, tap water at 0 ppm, 25% changes: 40 → 30 → 22.5 → 16.9 ppm. Two changes don’t reach 20 ppm; the third does. Alternatively a single 50% change goes straight from 40 to 20 ppm.
Real tanks keep producing waste between changes, so measured levels will be a bit higher than this ideal dilution. Match the temperature of the new water and treat tap water as your conditioner’s label directs.
Sources
Related tools: Tank volume by shape · Gallons ↔ liters & water weight · Substrate / gravel amount · Heater size & warm-up time · Product dose scaling