Aquarium lab
Aquarium water-change calculator
Calculate the percentage and litres to replace to lower nitrate by dilution.
- Result updates as you edit
- Units and limits shown
Starting scenario
- Net volume100L
- Current nitrate40mg/L
- Target nitrate20mg/L
- Replacement-water nitrate5mg/L
Aquarium inputs
Calculation workbench
Adjust the inputs. The result updates immediately, with the calculation method explained below.
Visual reference
How replacement works
The diagram shows the logic of removing aquarium water and adding prepared replacement water.
A water change is a comparison, not just a percentage
Measure first, calculate the volume, prepare compatible water and measure again under equivalent conditions.
- Measure first
- Aquarium water
- New water
- Target
- Verify afterwards
Calculation logic
What it calculates
Formula
(Current nitrate − target) ÷ (current nitrate − replacement-water nitrate) × 100, then applies that percentage to net volume.
Limits
It assumes complete mixing and no new nitrate during the change. It cannot reach a target at or below the replacement water.
Next step
Test both waters with the same method, match temperature and condition replacement water before adding it.