Aquarium water changes: how much, how often, and how

Short answer: for most stocked freshwater tanks, a 20 to 30 percent water change every one to two weeks is the sweet spot. It removes the nitrate and dissolved waste your filter cannot, tops up minerals, and keeps conditions stable. Always match the new water's temperature to the tank, and always treat tap water with a dechlorinator before or as it goes in. Heavily stocked tanks need more; lightly stocked, planted tanks can sometimes go a little longer. Regular and modest beats rare and drastic every time.

Why water changes matter

Your filter grows bacteria that turn ammonia into nitrite and then into nitrate. That is the safe end of the chain, but nitrate still accumulates — and the filter does not remove it. Dissolved organic waste builds up alongside it. A water change is the only routine way to physically take those out and replace what has been used up. No filter, plant or additive fully replaces the bucket.

How much, and how often

Use your stocking level as the guide:

• Average community tank — 20 to 30 percent every one to two weeks.
• Heavily stocked or big-messy fish — more often, or a larger percentage, to keep nitrate down.
• Lightly stocked, planted tanks — sometimes a little less, but do not skip them entirely.

Let your test kit settle the argument. If nitrate keeps climbing between changes, change more water or more often. The API Master Test Kit makes the trend easy to follow.

How to do a water change, step by step

• Turn off the heater and filter if the water level will drop below them.
• Siphon out your chosen amount with a gravel vacuum, drawing waste up out of the substrate as you go. A Fluval EasyVac handles most tanks; for a nano or betta tank a small hand siphon like the Marina Waste Remover is easier to control.
• Prepare the new water to roughly tank temperature, and add dechlorinator per the label.
• Refill gently so you do not blast the substrate or stress the fish.
• Turn everything back on and check the heater and filter are running.

Two things people get wrong

First, chlorine. Tap water in most of Alberta is treated with chlorine or chloramine, which harms fish and your filter bacteria. A dechlorinator every single time is non-negotiable — browse water conditioners.

Second, temperature and the bucket. Cold water dumped into a warm tank shocks fish, so match it by feel. And keep a dedicated bucket and gravel vacuum for the aquarium only — anything that has held soap or cleaner can kill a tank. You will find siphons, nets and brushes in aquarium maintenance.

Build the habit

A steady schedule is the single biggest thing behind a healthy tank. If you are still finding your feet, how to cycle a new aquarium explains the chemistry your water changes are managing, and your first aquarium puts the whole routine in context.

Written by the team at Tropical Fish Imports in Lethbridge, Alberta. Last reviewed August 2026.

Not sure how much to change on your particular tank? Tell us the size and what you keep and we will suggest a schedule — visit the shop, call 403-380-4506, or send us an email.