How to do a water change

Regular water changes are the single best thing you can do for your fish. They remove the waste your filter can't, and they only take about 20 minutes a week.

How much and how often?

For most beginner tanks, change about 25% of the water once a week. Heavily stocked tanks may need more, and lightly stocked tanks with lots of plants can often go a little longer.

The best way to know is to test nitrate. Aim to keep it under about 20–40 ppm. The water change calculator uses your test results to work out exactly how much to change.

What you need

• A gravel vacuum (siphon), or a hose kit that connects to a sink
• A bucket that's only ever used for the aquarium, because soap residue can kill fish
• Water conditioner
• A thermometer, to match the new water to the tank
• An old towel

Step by step

1. Unplug the heater and filter. The heater can crack if it's switched on out of the water, and some filters run dry when the level drops.
2. Wipe the glass first if it needs it, so the algae you scrape off gets siphoned out.
3. Start the siphon and push the gravel vacuum down into the substrate. Let it lift the dirt, then pull it out before the gravel gets sucked up. Work across part of the tank each week.
4. Stop when you've removed about 25%. Mark your bucket at the right level so you know when to stop.
5. Prepare the new water. Match the temperature to the tank within a couple of degrees, and add water conditioner. Most conditioners say to dose for the whole tank when you refill, so check the bottle.
6. Pour gently onto a rock, a plate or your hand so you don't blast the gravel and plants around.
7. Plug the filter and heater back in, and check that the filter starts up properly.

Mistakes to avoid

• Skipping the conditioner. Chlorine and chloramine in tap water harm fish and kill the good bacteria in your filter.
• Very cold or hot water. Sudden temperature swings stress fish.
• Rinsing filter media under the tap. Swish it in the old tank water you just removed instead, to keep the good bacteria alive.
• A huge change after months of no changes. In a neglected tank the water may be very different from your tap water. Change 10–20% a day over several days instead of all at once.
• Cleaning everything at once. Don't deep-clean the gravel, filter and decorations on the same day.

Related: Water change calculator · Cloudy aquarium water · Aquarium algae: types and fixes · How to clean a filter