Changing aquarium water is the single most important maintenance task, and done properly it takes about 15 minutes. The key rules: change about 25% weekly, match the temperature, and always dechlorinate the new water before it reaches the fish.

Here is the exact step-by-step.

Step by step

1. Unplug the heater if the water level will drop below it.

2. Vacuum out 25% of the water using an gravel vacuum, cleaning the substrate as you siphon into a bucket.

3. Prepare new water in a clean bucket at matching temperature — lukewarm, tested by hand.

4. Dechlorinate the new water with Seachem Prime or your conditioner, and wait 30 seconds.

5. Pour it in slowly, onto a plate or decor to avoid disturbing the substrate.

6. Plug the heater back in.

The rules that matter

• Change about 25% weekly — never more than 50% in one go except in emergencies.
• Match temperature within 1 °C to avoid shocking fish.
• Always dechlorinate — UK tap water contains chlorine or chloramine that kills fish and beneficial bacteria.
• Use a dedicated bucket that has never touched detergent.

Frequently asked questions

How much water should I change in my aquarium?

About 25% weekly for a normally stocked tank. Larger changes (up to 50%) are only for emergencies, as bigger swings in temperature and chemistry stress fish.

Do I need to dechlorinate water for every water change?

Yes. Every time tap water enters the tank it must be dechlorinated, because chlorine and chloramine are fatal to fish and kill the beneficial bacteria in your filter.

Should I turn off the heater during a water change?

Yes, if the water level will drop below the heater. Running a heater exposed to air can crack it or burn out the element.

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— Jaqline Smalling, FishForever