How often to change aquarium water

Why water changes matter

Water changes dilute nitrate, remove waste and dissolved organics, and keep the tank stable. How often you need to do them depends on tank size, how many fish you have, feeding, and filtration. There’s no single rule for every tank — but a few guidelines help.

A good starting point

For many tropical freshwater tanks: 10–25% once a week is a safe default. Use a gravel siphon to take water from the substrate and refill with dechlorinated water at a similar temperature. If your nitrate stays low and your fish look healthy, you might do a bit less or go every two weeks. If nitrate creeps up or the tank is heavily stocked, do more or change more often.

By tank size

Small tanks (5–10 gallons): Waste builds up fast. Weekly changes (or even twice a week in a heavily stocked nano) are common. Medium tanks (20–40 gallons): Weekly 20–25% is typical. Larger tanks: You can often do 20–30% weekly or every two weeks, depending on stock and nitrate. Test regularly so you see what your tank actually needs; an app like App-aquatic can remind you and log results so you spot trends.

When to change more often

Heavy stocking, big fish, or overfeeding = more waste. New tanks (cycling or just finished) often need extra changes until things settle. If nitrate is rising or fish look off, increase frequency or volume before adding more fish.

When you might change less

Lightly stocked, well-planted tanks with strong filtration sometimes get by on every-two-weeks or smaller weekly changes. Always let your test results and fish behaviour guide you — not a fixed calendar.

Quick takeaways

• Start with 10–25% weekly and adjust from there.
• Small tanks usually need weekly (or more); larger tanks can often do weekly or every two weeks.
• Use nitrate and fish health as your guide; set reminders so you don’t skip.