Water changes are the single most important maintenance task in fishkeeping. Done correctly, they remove nitrates, replenish minerals, and keep your fish healthy. Done incorrectly, they stress your fish, crash your cycle, or introduce disease. Here's how to do it right.
How Much and How Often?
The standard recommendation is 20–30% of tank volume once per week. Some situations require more:
- Heavily planted tanks with CO2: 50% weekly to replenish minerals
- Heavily stocked tanks: 30–40% twice weekly
- Tanks with bottom feeders (corydoras, plecos): gravel vacuum every change
- After medication: 30% every 48 hours until nitrates are below 20ppm
💡 Doing very large water changes (50%+) weekly is not better; it creates large parameter swings that stress fish more than elevated nitrates do.
The Right Way: Step by Step
Step 1: Prepare your new water first
Fill a clean bucket with tap water and add dechlorinator immediately. The new water should be the same temperature as your tank, within 1°C. Use a thermometer. Cold water shocks fish and can trigger ich.
Step 2: Turn off the heater
Always switch off your heater before water levels drop. Heaters exposed to air while hot will crack or burn out. Wait 15 minutes after turning it off before proceeding.
Step 3: Gravel vacuum the substrate
Push the gravel vacuum down into the substrate and let it pull up water and debris simultaneously. Work in sections, covering about 30% of the substrate per water change in a rotating pattern so you don't disturb the entire bed every time.
Step 4: Remove the water
Continue siphoning until you've removed your target amount.
Step 5: Add new water slowly
Pour the new water in slowly using a jug or bowl to avoid disturbing the substrate and fish. For larger tanks, a Python-style water changer connected to your tap allows simultaneous draining and filling.
Step 6: Restart equipment
Once the water level is back to normal, restart your heater and check the thermometer over the next hour to confirm temperature has stabilised.
What Not to Do
- Do NOT use hot water from the tap to warm the bucket; it may contain dissolved metals
- Do NOT clean your filter the same day as a water change; do them a week apart
- Do NOT vacuum the same area of gravel every week; rotate to preserve bacteria
- Do NOT skip water changes because the water "looks clear"; nitrates are invisible
- Do NOT use soap or detergent on any aquarium equipment, ever
Signs You're Not Changing Enough Water
- Fish gasping at the surface
- Loss of colour in usually bright fish
- Listlessness or fish hiding more than usual
- Algae growing faster than normal
- Nitrates consistently above 40ppm on your test kit
Find gravel vacuums, water conditioners, and test kits in our Maintenance range.
