Water change calculator

How much water should you change? Enter your tank size and nitrate test results to see exactly how many gallons to swap, today and every week.

Tank size (US gallons)

Nitrate now (ppm)

Target nitrate (ppm)

Nitrate in tap water (ppm)

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Water to change today

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Gallons to remove

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5-gallon buckets

Weekly routine

Nitrate rise in one week (ppm)

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Change each week

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Gallons each week

To find the weekly rise, test nitrate right after a water change and again a week later. The difference is your weekly rise.

How much water should I change?

For most tanks, about 25% once a week is a good routine. Heavily stocked tanks, or tanks where nitrate climbs fast, need more. Lightly stocked, planted tanks can sometimes get by with less.

The best guide is your nitrate test. Try to keep nitrate under 20 ppm, and never let it go above 40 ppm.

How this calculator works

Changing water dilutes nitrate. If you change half the water, nitrate drops by half (if your tap water has none). The calculator works out the exact share of water to remove to reach your target, and allows for any nitrate already in your tap water.

For the weekly routine, it finds the weekly change that stops nitrate from climbing past your target between water changes.

Is it safe to change a lot of water at once?

Big water changes are safe as long as the new water is treated with conditioner and is close to the same temperature. The danger comes from sudden swings in temperature or pH, not the amount of water. If nitrate is very high in a tank that's been neglected for a while, change about 25–30% a day over several days instead of all at once.

Water change tips

Unplug the heater and filter before you drain, so they don't run dry.
Use a gravel vacuum to clean part of the substrate while you remove water.
Add conditioner for the full tank volume when refilling with tap water.
Keep a bucket that's only ever used for the aquarium.