How to do an aquarium water change, step by step

What you need

• A gravel vacuum (siphon) sized for your tank. It removes water and pulls debris out of the substrate at the same time.
• One or two buckets used only for the aquarium. Never wash them with soap or detergent.
• Water conditioner (dechlorinator) that neutralises chlorine and chloramine.
• A thermometer to match the new water to the tank.
• An algae scraper or magnet and an old towel for drips.

Before you start, know how many litres or gallons you are going to remove. For a 100-litre tank, a 25% change is 25 litres, about two and a half 10-litre buckets. Measuring once means you do not have to guess every week.

Step by step

1. Switch off the equipment

Unplug the heater and the filter before the water level drops. A heater left running out of the water can overheat and crack, and a filter that sucks air can run dry and damage its motor. Leave the fish in the tank: catching them causes more stress than the water change itself.

2. Clean the glass first

Scrape algae off the inside of the glass before you siphon. Whatever comes loose will settle and be removed with the old water instead of floating around afterwards.

3. Siphon and vacuum the substrate

Start the siphon and push the wide end of the gravel vacuum into the gravel. Let it fill with debris, then lift it slightly so the gravel falls back while the dirty water flows into the bucket. Work across the open areas of the substrate in sections. Over sand, hover just above the surface instead of digging in. In a planted tank, stay away from the roots and clean only open patches.

4. Stop at your target volume

For most tanks that is 20–30% of the water. Keep an eye on the bucket rather than the tank so you do not overshoot. Do not empty the tank completely: a full water change and a scrubbed-clean tank remove the stability your fish depend on.

5. Keep some old water for the filter

If filter maintenance is due, set aside a bucket of the water you just removed. That is the water to rinse the filter sponge in, never tap water. See how often to clean an aquarium filter for details.

6. Prepare the new water

Fill a clean bucket from the tap and check the temperature against the tank thermometer. Aim to get within a degree or two. Add dechlorinator according to the product label, dosing for the volume of new water you are adding, or for the whole tank if the label says so. For saltwater tanks, mix the salt to the correct salinity in advance; do not add fresh tap water to a marine tank.

7. Refill gently

Pour slowly, onto your hand, a plate or a decoration, so you do not blast the substrate or uproot plants. Fill back up to the original level.

8. Switch everything back on

Restart the filter, wait until it is full of water and running, and then plug in the heater. Check after a few minutes that the flow is normal and the fish are behaving as usual.

9. Write down the date

It sounds trivial, but this is the step people skip, and it is the one that tells you next week whether you are on schedule.

Common mistakes

• Forgetting the dechlorinator. Chlorine and chloramine in tap water harm fish and the filter's beneficial bacteria.
• Cold or hot new water. A big temperature jump stresses fish and can trigger disease.
• Soap anywhere near the tank. Buckets, sponges and hands should be soap-free.
• Cleaning everything on the same day. A deep gravel clean, a fully scrubbed filter and a large water change together can knock back the bacteria colony. Spread heavy cleaning over different weeks.
• Removing the fish. Netting them is usually more stressful than the water change.
• Leaving the heater on. Unplug it before the water level drops.

A note on saltwater and sensitive tanks

Marine tanks follow the same idea with two differences: the new water is mixed with salt to the right salinity before it goes in, and many reef keepers prefer smaller, steady changes of around 10% because corals react to swings in chemistry. Tanks with soft-water species or shrimp also benefit from small, regular changes rather than occasional big ones. If your tap water is very different from what your fish need, prepare and test the new water in advance.

How long it takes

For a 60–200 litre tank, a routine 25% change with gravel vacuuming takes about 20–40 minutes once you have the equipment in one place. Small tanks are faster but need it more often; large tanks may be easier with a hose that connects to the tap, so you are not carrying buckets.

How often should you do it?

For most freshwater tanks, once a week. Heavily stocked tanks and messy fish such as goldfish may need more, lightly stocked planted tanks less. The best guide is a nitrate test right before each change. There is a full breakdown in how often to change aquarium water.

Remembering the next one

A water change done every week keeps a tank stable. One done "when I remember" does not. Siphon keeps the date for you: tap "Changed water today" when you finish, and the app counts the days and reminds you when the next change is due. In Siphon Pro you can add the other jobs from this guide, such as filter rinse, glass cleaning and water tests, each on its own interval, and log your test results.

This guide covers typical freshwater tanks. For special setups, follow the advice for your species and the instructions on your water conditioner and equipment.