Convert a chosen percentage into a real amount of water without confusing the maths with the maintenance decision.
If an aquarium actually contains 120 litres and you choose a 25% change, the arithmetic amount is 30 litres. This tool answers that quantity question only; it does not invent a universal maintenance schedule.
A tank sold as 120 litres may contain less once substrate, hardscape, equipment and the air gap are included. If the real water quantity is 105 litres, 25% is 26.25 litres, not 30.
The percentage calculation is only as good as the water-volume figure you start with.
The appropriate maintenance amount varies with stocking, feeding, plant growth, filtration, tap-water characteristics and measured water quality. The RSPCA notes that water can look clear while still containing harmful waste and recommends water testing rather than relying on appearance.
Test the aquarium, decide on the maintenance action using appropriate care guidance, calculate the amount from actual water volume, prepare replacement water appropriately and re-test when correcting a problem. Keeping those steps separate makes the calculator useful without pretending it is a care protocol.
| Actual water volume | 10% | 20% | 25% | 30% |
|---|---|---|---|---|
| 60 L | 6 L | 12 L | 15 L | 18 L |
| 100 L | 10 L | 20 L | 25 L | 30 L |
| 180 L | 18 L | 36 L | 45 L | 54 L |
The table is arithmetic, not a maintenance prescription. The right schedule and percentage depend on the aquarium, test results, stocking, feeding, filtration and the needs of the animals.
Clear water can still contain dissolved substances that matter to fish. The RSPCA fish-environment guidance emphasises testing and regular partial water changes rather than judging an aquarium only by how clear it looks. UK ornamental-aquatic guidance from OATA is another useful source of care context.
This calculator deliberately separates the decision how much should I change? from the arithmetic how many litres is that percentage?. Once you have chosen an appropriate percentage, the maths should be unambiguous.