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How the calculation works
Replacement volume equals actual system water volume × change percentage ÷ 100. The remaining volume is actual water minus the removed amount. A change is assumed to remove and replace equal volumes; this calculator does not model evaporation top-off or a changing operating waterline.
Start with actual water, including sump water in a connected system and excluding estimated solid displacement. Use the volume calculator if you only have internal dimensions. The unit selector converts the entered quantity, so switching between liters and US gallons preserves the same water amount.
Choose the change percentage for your aquarium’s condition and inhabitants. Matching appropriate temperature and chemistry, treating replacement water correctly and retesting afterward are separate care decisions. The arithmetic alone cannot choose a safe rate, diagnose a water-quality problem or predict all dissolved substances.
Worked example
For 100 liters of actual water and a 25% change, prepare 25 liters of replacement water; 75 liters remains. For 100 US gallons, the same percentage means 25 US gallons, approximately 94.64 liters.
Common questions
Do I dose conditioner for the whole aquarium?
Follow the particular product label and addition method. Some labels distinguish treating new water separately from dosing directly into the aquarium. The dose calculator makes that basis explicit.
How much change will lower nitrate to a target?
Use the nitrate planner. It includes the nitrate already present in replacement water and the difference between one large change and repeated smaller changes.