Aquarium Water Change Calculator
How many gallons to swap in a water change — from your tank size and the percentage you change each time.
How it works
Water per change = tank volume × change % · weekly total = water per change × changes per week
Regular partial water changes dilute nitrate and replenish minerals without shocking fish by swapping the whole tank. The volume to replace is simply the tank size times the percentage you remove; a common maintenance routine is 20–30% weekly, adjusted up for heavily stocked tanks and down for lightly stocked or planted ones. Knowing the exact gallons makes it easy to pre-treat replacement water with the right dose of dechlorinator.
Worked example
On a 40-gallon tank a 25% change removes and replaces 40 × 0.25 = 10 gallons (about 38 liters). Doing that once a week means treating and swapping 10 gallons of fresh, temperature-matched water each time.
Frequently asked questions
How much water should I change and how often?
A weekly 20–30% change suits most community tanks. Increase the percentage or frequency for crowded tanks or high nitrate readings, and decrease it for lightly stocked or mature planted tanks.
Do I need to dechlorinate the new water?
Yes, if you use tap water. Chlorine and chloramine harm fish and beneficial bacteria, so dose a conditioner for the number of gallons you are adding — the per-change figure above tells you exactly how much.
Should the new water match the tank temperature?
Match it as closely as you can. Large swings stress fish, so aim for within a degree or two of the tank when refilling.
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This calculator is for educational purposes. Double-check important results before acting on them.