Make every change measurable.
Choose a target percentage, then compare the amount removed and added. See the replacement volume and the final water balance.
Water change
The percentage is your input, not a husbandry recommendation. One drain-and-refill event; no continuous water-exchange model.
THE GEOMETRY, MADE VISIBLE
V × p ÷ 100
Final water = starting water − removed + added.
Target amount to replace
- Water after target removal
- 38.7 L
- Actual removed percentage
- 25%
- Water after removal + addition
- 51.6 L
- Added minus removed
- 0 L
Equal amounts removed and added
Your inputs and results are included in print and CSV output.
Use internal straight-sided dimensions. This calculator does not determine safe stocking, medicine doses or structural load.
A little more certainty around the tank.
Tank volume
Use internal measurements and the actual waterline. See full capacity, gross water volume and your displacement-adjusted estimate.
Open calculatorSubstrate amount
Calculate substrate volume from the footprint and depth. Use a front-to-back slope for rectangular tanks and your own bulk density for a weight estimate.
Open calculatorTop-up & water level
Convert a vertical water-level difference to litres and millilitres using the inside footprint of your tank.
Open calculatorAre US and imperial gallons interchangeable?
No. Aquakind labels them separately: one US liquid gallon is 3.785411784 L; one imperial gallon is 4.54609 L. Always check which gallon a product label uses.
Why is actual water lower than the advertised tank size?
Glass thickness, a waterline below the rim, substrate and submerged decorations can reduce the water present. The tool uses your internal measurements and only the displacement you enter.
Can I use this for a bow-front tank or a horizontal cylinder?
No. The supported geometries are a straight-sided rectangular tank and a vertical circular cylinder. Irregular tanks, sumps, pipework and external filters are not included. Measure those volumes separately.
Will the calculator tell me how much water to change?
You choose the percentage. The calculator translates it to a volume and checks the amount removed and added. It does not prescribe a maintenance schedule, fish stocking, treatment or water chemistry.
Is substrate density the same as stone density?
No. Bulk density describes material including the space between grains. Material density describes the grains themselves. Use a suitable measured or manufacturer-provided bulk density when estimating a purchase mass.