Practical keeper tools
What remains after several water changes?
Multiply the remaining fractions, one step at a time. This is an arithmetic model, not a measurement of your aquarium.
Enter each change in order
Each percentage refers to the well-mixed water present immediately before that step. The default example is 20% followed by 30%.
| Step | Water changed | Original amount remaining |
|---|---|---|
| 1 | 20% | 80% |
| 2 | 30% | 56% |
Working: multiply the fractions left after each change. For 20% then 30%: 0.8 × 0.7 = 0.56, or 56%.
Assumes complete mixing and replacement water containing none of the tracked substance. Ignores binding, filtration, evaporation, chemical changes and other additions. A modelled zero or near-zero result does not establish real-world clearance. No redosing or livestock-return advice.