Water change calculator
How much water should you actually change? Enter your tank volume and where your nitrate is versus where you want it, and get the exact percentage and gallons to swap out.
A common target is keeping nitrate under 20 to 40 ppm. Test your tap water too: if it already contains nitrate, a water change cannot pull the tank below that level.
The math
A water change dilutes what is in the tank. Replacing a fraction of the water with nitrate-free water leaves current × (1 − fraction changed), so to get from your current nitrate down to the target you change 1 − (target ÷ current) of the volume. The calculator turns that into a percentage and a number of gallons.
Let the test kit, not the calendar, set the schedule
About 25% a week is a good default, but the honest schedule is whatever keeps your nitrate in range between changes. Test, adjust, repeat. A heavily stocked tank needs more; a lightly stocked, planted one needs less. A liquid test kit gives you the real number to work from.
The tap-water catch
You cannot dilute below what your tap water already contains. If your tap runs, say, 20 ppm nitrate, no amount of water changing gets the tank under 20. Test your tap so you know the floor you are working against.
After the change
Always refill with dechlorinated, temperature-matched water. Work out the exact conditioner dose with the conditioner dosing calculator.
Source
- Standard dilution: final concentration = initial × (1 − fraction replaced), assuming nitrate-free replacement water.
Common questions
How much water should I change in my aquarium?
About 25% weekly suits most community tanks, but the honest amount is whatever keeps your nitrate in range. If nitrate is high, the calculator shows the exact percentage to change to reach your target.
How do I lower nitrates with a water change?
A water change dilutes nitrate: replacing half the water roughly halves it. Enter your current and target nitrate for the percentage to change, and remember you cannot go below your tap water's own nitrate level.
Can you change too much water at once?
Large single changes can swing temperature and chemistry faster than fish like, so routine changes are usually kept to 25 to 50%. If the calculator suggests more than 50%, split it into two changes a day or two apart.
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