How to Do a Water Change Without Stressing Your Fish
Regular water changes keep your aquarium safe, but done carelessly they shock fish with sudden temperature and chemistry swings. This step-by-step guide shows how much water to swap, how often, and how to treat and temperature-match replacement water so your fish barely notice the change.
Quick answer
A good water change is small, regular, and temperature-matched. Swap 20-30% of the tank volume every week using dechlorinated water at roughly the same temperature, siphon the substrate gently as you drain, and refill slowly. Done this way, your fish stay calm and your water parameters stay stable.
Regular water changes keep your aquarium safe, but done carelessly they shock fish with sudden temperature and chemistry swings.
Why water changes matter
Even a fully cycled filter cannot remove everything. Nitrate, dissolved organics, and trace waste build up steadily between changes, and the only reliable way to dilute them is to physically remove old water and add clean water. Skipping changes lets nitrate creep up and pH slowly drift down, which stresses fish over weeks even when the tank looks clear.
How much and how often
For a normally stocked tank, 20-30% weekly is the sweet spot. Lightly stocked or planted tanks can go slightly longer; heavily stocked tanks or those with messy eaters like goldfish may need twice weekly. Let your test kit guide you: if nitrate sits above 40 ppm just before a change, do more or larger changes.
Step by step
Work calmly and keep the fish in the tank the whole time.
Vacuum the substrate gently in sections so you remove waste without uprooting plants.
- Unplug the heater and switch off the filter if the water level will drop below the intake.
- Use a gravel siphon to vacuum the substrate section by section, draining old water into a clean bucket until you have removed 20-30%.
- Fill a second clean bucket with tap water, add the correct dose of dechlorinator, and adjust the temperature.
- Pour or pump the treated water back in slowly, aiming the flow at the glass or a plate so you do not blast the substrate.
- Restart the filter and heater, and check that everything is running.
Treat and temperature-match the new water before it ever touches the tank.
Temperature and dechlorinator
The two things that actually shock fish are chlorine or chloramine in untreated tap water and a sudden temperature swing. Add a water conditioner that neutralises both chlorine and chloramine at the dose on the bottle, and float a thermometer in your refill bucket so the new water is within a degree or two of the tank. In humid, high-rise flats where tanks sit near air-conditioning, cold refill water is a common hidden cause of stress.
Quick FAQs
How long does a water change take? For a typical 60-100 litre tank, about 15-20 minutes once you have a routine.
Do I need to remove the fish? No. Removing and re-netting fish causes far more stress than a partial change ever will.
Can I use bottled or filtered drinking water? You can, but it is expensive and often stripped of minerals fish need. Dechlorinated tap water is usually the better choice.
What if I skipped a few weeks? Do not fix it with one giant change. Spread several 20-25% changes over a few days so parameters shift gradually.