How much water should I change in my aquarium?

For most tanks, 15–35% per week depending on stocking: around 15% for lightly stocked, 25% for a typical community, and 35% or more for heavily stocked tanks. Enter your volume and a percentage above to see the exact amount in gallons or liters.

How often should I do water changes?

Weekly is the standard for most aquariums. Smaller, regular changes keep nitrate and other waste stable better than occasional large ones. Heavily stocked tanks may benefit from twice-weekly changes; lightly stocked, heavily planted tanks can sometimes stretch longer.

How do I lower high nitrate?

Dilute it with water changes. Because each change only swaps part of the water, it takes several to make a big dent — this calculator works out how many changes of a given size reach your target. Pair that with less feeding, more plants, and not overstocking to keep nitrate down long-term.

Is it better to do one big change or several small ones?

Several smaller changes are usually gentler. A very large single change can swing temperature and chemistry enough to stress fish. Frequent partial changes reach the same target with less disruption, though a one-off larger change is fine for an emergency reset.

Do I need to treat the new water?

Yes. Most tap water contains chlorine or chloramine that harms fish and filter bacteria, so use a dechlorinator. Match the new water's temperature to the tank, and check whether your tap water itself carries nitrate, which limits how low changes can take you.

Free aquarium calculator

Aquarium Water Change Calculator & Scheduler

See exactly how much water to swap and plan a routine. Enter your tank volume and a change percentage to get the volume per change — and optionally plan how many changes it takes to lower nitrate to a target.

Net water volume

Base changes on the net volume — the water your fish actually swim in.

Change percentage

Pick a maintenance level or type your own percentage.

Lower a parameter (optional)

Plan how many changes it takes to dilute nitrate (or any dissolved level) down to a target.

Current level

ppm

Target level

ppm

Per water change

Replace each time

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Results are estimates. Dilution math assumes the new water has none of the parameter you're lowering — check your tap water, since some sources carry nitrate. Match new water's temperature and treat it for chlorine before adding.

How the water change calculator works

Enter your tank's net water volume and the percentage you plan to change, and the calculator shows exactly how much water to remove and replace each time. If you're battling high nitrate or another dissolved level, add your current and target readings and it works out how many changes — and roughly how many weeks — it takes to get there.

The method

The volume per change is just net volume × percentage. Lowering a dissolved parameter is dilution: each partial change replaces a fraction of the water, so after n changes of fraction p the level is current × (1 − p)ⁿ. Inverting that gives the number of changes to reach a target. A bigger single change clears it faster, but many keepers prefer smaller, more frequent changes to avoid shocking fish.

Worked example

On a 200-liter tank a 25% change removes 50 liters. If nitrate sits at 80 ppm and you want it under 20 ppm, 25% changes get you there in about 5 changes (80 → 60 → 45 → 34 → 25 → 19 ppm) — roughly five weeks at one a week, or faster if you do a couple early on. A single 50% change would take about half as many.

Building a routine

A lightly stocked tank usually stays healthy with a 15% weekly change; a typical community wants around 25%; a heavily stocked or messy tank often needs 35% or more. Consistency matters more than size — a steady weekly change keeps nitrate from ever climbing, which is easier on fish than occasional big resets.

Frequently asked questions

How much water should I change in my aquarium?: For most tanks, 15–35% per week depending on stocking: around 15% for lightly stocked, 25% for a typical community, and 35% or more for heavily stocked tanks. Enter your volume and a percentage above to see the exact amount in gallons or liters.
How often should I do water changes?: Weekly is the standard for most aquariums. Smaller, regular changes keep nitrate and other waste stable better than occasional large ones. Heavily stocked tanks may benefit from twice-weekly changes; lightly stocked, heavily planted tanks can sometimes stretch longer.
How do I lower high nitrate?: Dilute it with water changes. Because each change only swaps part of the water, it takes several to make a big dent — this calculator works out how many changes of a given size reach your target. Pair that with less feeding, more plants, and not overstocking to keep nitrate down long-term.
Is it better to do one big change or several small ones?: Several smaller changes are usually gentler. A very large single change can swing temperature and chemistry enough to stress fish. Frequent partial changes reach the same target with less disruption, though a one-off larger change is fine for an emergency reset.
Do I need to treat the new water?: Yes. Most tap water contains chlorine or chloramine that harms fish and filter bacteria, so use a dechlorinator. Match the new water's temperature to the tank, and check whether your tap water itself carries nitrate, which limits how low changes can take you.