Water change calculator
Single change
Use the same units for all readings, such as ppm.
Water to replace
Not calculated%
Repeated changes
Changes needed
Not calculatedchanges
Test replacement water and use the same units for all readings. Dechlorinate and temperature-match it. Large changes can shift pH and KH; test the pond again afterward.
Dilution method
C after = C before × (1 − x)
x = 1 − Ct / C0 (replacement water at 0)
C after = Cr + (C before − Cr) × (1 − x) (replacement water at Cr)
x = 1 − (Ct − Cr) / (C0 − Cr)
C_n = Cr + (C0 − Cr) × (1 − x)^n
n = ln((Ct − Cr) / (C0 − Cr)) / ln(1 − x) rounded up to a whole change
Assumes complete mixing, a constant replacement-water reading, and no production or consumption of the substance between changes. The model approaches the replacement-water concentration; it cannot reduce a reading below that level.
Routine changes are commonly 10 to 25%. If a reading keeps rising, address the source as well as changing water. Evaporation leaves dissolved salt behind; use an actual water change to reduce salt concentration.