Preparing calculator preferences…
How the calculation works
Use the same nitrate basis for all three readings: mg/L as NO₃, often labeled nitrate ppm by aquarium kits. Do not mix nitrate-nitrogen (NO₃-N) readings with nitrate values. After a fraction f is replaced, concentration is Cnew = Ccurrent × (1 − f) + Creplacement × f.
For n fully mixed equal changes, concentration is Creplacement + (Ccurrent − Creplacement) × (1 − f)ⁿ. The planner solves for f and lists the predicted reading after each change. Cumulative replacement water can exceed the tank’s volume because later changes remove some previously added water.
This is dilution only: no feeding, biological nitrate production, plant uptake, denitrification or incomplete mixing is modeled. A target below replacement-water nitrate is unreachable by dilution. A target equal to replacement-water nitrate requires full replacement mathematically. Neither result recommends an abrupt 100% change; retest and choose a livestock-appropriate plan.
Worked example
Starting at 80 mg/L, targeting 20 mg/L, with replacement water at 5 mg/L requires an 80% single change. Two equal changes instead require about 55.28% each: the first predicts 38.54 mg/L and the second 20 mg/L. For 100 liters, cumulative replacement is about 110.56 liters.
Common questions
Why can I not reach 2 mg/L with 5 mg/L replacement water?
Dilution approaches the replacement concentration. It cannot pass below it when starting higher. Test a suitable lower-nitrate source rather than increasing a mathematically impossible change.
Why is measured nitrate higher than predicted?
Production between changes, test uncertainty, actual-volume errors or incomplete mixing can change the result. Retest replacement water and the aquarium instead of assuming the estimate is a diagnosis.