PondTools

Water change calculator

These numbers are typed in. Trace your pond once and every calculator uses its measured volume and depths.

Measure my pond

One change

Use the same units for every reading, such as ppm.
Water to replace
Not calculated%

A series of smaller changes

Changes needed
Not calculatedchanges
How it works

Test the replacement water and use the same units for every reading. Dechlorinate it and match its temperature to the pond. Large changes can shift pH and KH, so test the pond again afterward.

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

This assumes the water mixes completely, the replacement water's reading stays the same, and nothing in the pond adds or removes the substance between changes. A series of changes only approaches the replacement water's reading and can never take the pond below it.

Routine changes are usually 10 to 25%. If a reading keeps climbing, find and fix the source as well as changing water. Evaporation leaves dissolved salt behind, so topping up does not lower salt. Only a real water change does.

Want the precise number?

Type your measurements in above for a quick estimate. For the real volume, trace your pond on a photo or satellite image and set its depths. PondTools measures the actual outline, so dosing, liner and pump sizes come out right.

Measure my pond precisely