Aquarium Water Change Calculator

Calculate how many gallons to replace for a partial water change, based on your tank's total volume and your target change percentage.

Tank Volume (gallons)

Change Percentage (%)

The percentage, and what it dilutes

gallons to replace = volume × percent / 100
gallons left in    = volume - gallons replaced

A 25% change on 55 gallons is 13.75 gal out and 41.25 gal left in the tank. The same fraction applies to everything dissolved: nitrate at 40 ppm drops to 30 ppm, because you removed a quarter of it. Changes compound rather than add - two 25% changes leave 0.75 × 0.75 = 56% of the original nitrate, not half.

Doing the change

Use your real volume. A nominal 55 holds nearer 48 gallons once substrate, rock and freeboard are counted, so 13.75 gal is closer to a 29% change than a 25% one.
Match temperature and treat for chlorine before the new water goes in. Dose the conditioner for the volume being added, not the whole tank, unless the label says otherwise.
Vacuum the substrate as you drain, so the water you remove carries waste out with it rather than just being water.
10-25% weekly suits most stocked freshwater tanks. Heavy stocking or a fish-in cycle needs more and sooner; a lightly stocked planted tank needs less.

Water changes dilute nitrate and replace minerals - they do not cycle a tank. Ammonia and nitrite are consumed by bacteria that need weeks to establish on your filter media, and a tank must finish cycling before it is stocked. If a long-neglected tank has drifted a long way on pH or hardness, get back to normal with several moderate changes rather than one huge one, so the chemistry moves gradually.

Frequently asked questions

How many gallons is a 25% water change on a 55 gallon tank?

13.75 gallons, leaving 41.25 in the tank - just under three 5 gallon buckets.

How much does a water change reduce nitrate?

By the same percentage as the change, provided the new water contains none. A 50% change on 40 ppm gives 20 ppm; getting from 80 ppm to 20 ppm needs 75% of the water replaced, best done as two or three separate changes.

How often should I change aquarium water?

Weekly is the usual rhythm, at 10-25%. Let your test kit decide the size: if nitrate is climbing week on week, change more or more often.