Leash & BowlCaring for animals, costed out before you have to decide

Planning guides

Aquariums · Water Quality

How Much Aquarium Water to Change, and How Often

Regular partial water changes remove the nitrate filters cannot. A weekly routine, how much to swap, and why huge changes can do more harm than good.

By Leash & Bowl Editorial 6 min read
A plecostomus catfish resting on aquarium gravel
On this page

A running filter makes a tank look clean, so water changes are easy to put off. But the filter does not remove everything. It converts dangerous ammonia into nitrate, which builds up week after week until you take water out.

Key figures

  • 15 to 25 percent Weekly partial betta water change, per PetMD
  • 80 percent Example: abrupt change that can shock fish

Why water changes matter

The nitrogen cycle ends in nitrate. It is far less toxic than ammonia or nitrite, but it accumulates, along with dissolved waste and hormones fish release. Replacing part of the water with fresh, treated water keeps all of that in check.

A simple weekly routine

PetMD’s betta care sheet recommends that each week you:

  1. Test the water for ammonia, nitrite, nitrate and pH.
  2. Siphon the substrate with a gravel vacuum.
  3. Change 15 to 25 percent of the water.

The same rhythm works for most small community tanks. Heavily stocked tanks may need more.

Doing it right

  • Treat new water with a conditioner for chlorine and chloramine.
  • Match temperature closely.
  • Do not clean everything at once. Rinse filter media in old tank water only, and not on the same day as a big gravel clean.
  • Wash your hands before and after. The CDC advises wearing gloves if you have cuts.
Big changes after long gaps

If a tank has gone months without a water change, the chemistry has drifted. A sudden 80 percent change can shock the fish. Return to normal with several smaller changes over a couple of weeks.

The test-kit guide explains the readings used to assess water. The tap-water chloramine guide covers the replacement water entering the aquarium.

Questions people ask

How often should I change aquarium water?

Weekly partial changes are a common routine. PetMD suggests 15 to 25 percent a week for a betta tank.

Should I remove the fish during a water change?

No. Leave fish in the tank. Removing them causes more stress than a partial change.

Do I need a water change if I have a filter?

Yes. Filters convert waste to nitrate but do not remove it. Only water changes do.

Can I change 100 percent of the water?

Avoid it. A full change removes stable conditions and can shock fish. Partial changes are safer.

Where these figures come from

  1. Betta Fish Care Sheet PetMD · petmd.com
  2. Ammonia in Aquatic Systems University of Florida IFAS Extension · edis.ifas.ufl.edu
  3. Fish, Healthy Pets Healthy People Centers for Disease Control and Prevention · cdc.gov

Figures last checked September 2026.

Researched and written by

Leash & Bowl Editorial

Every figure in this guide comes from a published source, linked above, and was checked before publication. No affiliate links and no sponsored posts.

How we work and who is behind it

More in Aquariums