Water Change Calculator
Work out how many litres to change to hit your target percentage.
How it works
Water to change (L) = Tank water volume (L) × (Percentage ÷ 100). To dilute a parameter, required % change ≈ (1 − target ÷ current) × 100.
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Formula & Method
Water to change (L) = Tank water volume (L) × (Percentage ÷ 100). To dilute a parameter, required % change ≈ (1 − target ÷ current) × 100.
Example Calculation
For a 40% change on a 60 L tank: 60 × 0.40 = 24 L. To halve a nitrate reading you'd change about 50% of the water.
Frequently Asked Questions
How often should I change aquarium water?
Weekly at 25-30% suits most moderately stocked freshwater tanks. Heavily stocked tanks need 40-50% weekly. The right frequency is whatever keeps nitrate under about 20 ppm between changes, which you determine by testing.
Can I change 100% of the water?
Almost never necessary and usually harmful. It removes the buffering capacity and beneficial bacteria held in the water column and causes a sharp parameter swing. Even after a serious problem, staged 30-50% changes are safer.
Does Australian tap water contain chloramine?
Most Australian capital city supplies use chloramine rather than free chlorine. Chloramine does not evaporate when water is left standing, so a conditioner that specifically neutralises chloramine is required.
What temperature should replacement water be?
Within about 1-2 C of the tank. In a Melbourne winter, mains water can be below 12 C, which is cold enough to shock fish and trigger disease. Mix hot and cold at the tap or pre-heat the change water.
Should I turn the heater off during a water change?
Yes, if the water level will drop below the heater. Modern heaters usually survive brief exposure, but running one in air is the fastest way to crack it.