How to Do an Aquarium Water Change (and How Often)

Regular water changes are the most important thing you can do for healthy fish. Here is exactly how to do one safely, and how much and how often.

Why water changes matter

Even a perfect filter cannot remove everything. Nitrate, dissolved organics and other waste slowly build up between changes, and the only way to remove them is to take out old water and replace it with fresh. Regular water changes are the backbone of a healthy tank - more important than any gadget.

How much and how often

For most community tanks, change 20-30% of the water once a week. Heavily stocked or messy tanks may need more; lightly stocked planted tanks can sometimes get away with less. Consistency beats size - small regular changes are far better than a big change once in a while. Use our water change calculator to work out the litres.

Step by step

1. Turn off the heater and filter. 2. Use a gravel vacuum (siphon) to remove the right amount of water while cleaning debris from the substrate. 3. Prepare the new water: match the temperature to the tank and add a dechlorinator to neutralise the chlorine and chloramine in Australian tap water. 4. Slowly pour or pump the treated water back in. 5. Turn the heater and filter back on.

Common mistakes to avoid

Never add untreated tap water - chlorine kills both fish and your filter bacteria. Don't change too much at once on an established tank (it can shock fish). Don't forget to temperature-match. And don't clean your filter at the same time as a big water change, or you may disturb too much beneficial bacteria at once.

Frequently Asked Questions