How Often Should You Clean a Fish Tank?

"Cleaning" a fish tank mostly means changing part of the water to remove the waste that builds up. The beneficial bacteria that keep fish alive live in the filter and gravel, so the goal is to remove waste without wiping out those bacteria.

Fish tank cleaning schedule

Chewy's veterinarian-reviewed guidance recommends weekly water tests and regular water changes; for goldfish, which produce a lot of waste, Chewy suggests a 25% water change every week. Aquarium Co-Op prefers changes of around 30%, avoiding drastic changes that stress fish.

How to know when water needs changing

Aquarium Co-Op's approach is to test nitrates regularly and change water before they reach about 40 parts per million, the level generally considered safe for most fish. If your nitrates rise 10 ppm a week, a weekly change keeps you safe; a lightly stocked, planted tank may go longer.

Ways to need fewer water changes:

• Feed less, or keep fewer fish.
• Use a bigger tank, so waste is diluted.
• Add live plants, which use nitrate as they grow.

Step-by-step partial water change

1. Unplug the heater and filter if the water level will drop below them.
2. Use a gravel vacuum to siphon water out while cleaning the substrate.
3. Remove about 25–30% of the water.
4. Treat the new tap water with a water conditioner (dechlorinator).
5. Match the temperature to the tank water.
6. Refill slowly, plug equipment back in and check the temperature.

Mistakes to avoid

• Changing 100% of the water or scrubbing everything at once: it removes beneficial bacteria and shocks fish.
• Washing filter media under tap water: chlorine kills the bacteria.
• Adding untreated tap water.
• Skipping tests and waiting until the water looks dirty; ammonia and nitrite are invisible.

How often to clean a betta or goldfish tank

Bettas in a filtered tank of at least 5 gallons need regular partial water changes and weekly tests. Goldfish are messy: Chewy recommends at least 20 gallons for one goldfish plus 10 per additional fish, a strong filter and 25% weekly water changes. Unfiltered bowls need far more frequent changes and are not recommended.

Find pet stores near you for test kits, conditioners and gravel vacuums.

Sources

1. Aquarium Co-Op: How often do you have to change water in a fish tank?
2. Chewy: How long do goldfish live? (2026)
3. Chewy: The 10 best pet fish for beginners (vet-reviewed, 2026)
4. PetMD: How to take care of goldfish (2024)