The short answer
Change about a quarter of the water every week.
That one habit keeps most tanks healthy. A quarter (25%) is enough to carry the waste away, and small enough that the fish barely notice.
If your tank is lightly stocked and full of plants, you can stretch to every two weeks. If it is crowded, or holds messy fish such as goldfish, change more.
How much and how often, by tank type
| Your tank | How much | How often |
|---|---|---|
| Normal community tank | 25% | Every week |
| Lightly stocked, heavily planted | 25% | Every two weeks |
| Betta in 5 gallons or more | 25–30% | Every week |
| Goldfish | 40–50% | Every week |
| Big cichlids (oscars and friends) | 40–50% | Every week |
| Shrimp tank | 10–20% | Every week |
| Small tank under 5 gallons | 30–50% | Twice a week |
| New tank that is still cycling | 25–50% | Whenever ammonia or nitrite reads above zero |
Small tanks need more changes, not fewer. There is so little water that waste builds up fast.
Why water changes matter
Fish make waste. Bacteria in the filter turn that waste into a much safer substance called nitrate. This is the nitrogen cycle.
But nitrate does not go away. It builds up, week after week, like a bath that is never drained. At high levels it weakens fish, feeds algae, and shortens lives.
Topping up water that has evaporated does not help. Only pure water evaporates. The waste stays behind and gets stronger. The only fix is to take old water out and put new water in.
Let a test kit decide
The best schedule is the one your own tank tells you. Buy a nitrate test kit and test just before a water change.
- Under 20 ppm: excellent. Your routine is working.
- 20 to 40 ppm: fine for most fish. Stay on schedule.
- Over 40 ppm: change more water, or change it more often.
The water change calculator turns your nitrate reading into the exact amount to change.
How to change the water, step by step
- Unplug the heater and filter. A heater that runs out of water can crack.
- Siphon the old water out with a gravel vacuum. Push the tube into the gravel as you go. It sucks up dirt and leaves the gravel behind.
- Fill a bucket with new tap water that feels the same temperature as the tank. Check with a thermometer until you can judge it by hand.
- Add water conditioner (dechlorinator) to the new water. Tap water has chlorine in it to kill germs. It also kills fish and the good bacteria in your filter. Conditioner makes it safe in seconds.
- Pour the new water in slowly, onto a rock or a plate so it does not dig a hole in the gravel.
- Plug everything back in. Check the filter starts running again.
A bucket used for the fish tank should be used for nothing else. Soap and cleaning sprays are deadly to fish, even in tiny traces.
Three mistakes to avoid
- Changing all the water at once. A 100% change shocks the fish with a sudden jump in temperature and water chemistry. Even in an emergency, two changes of 50% a few hours apart are safer.
- Washing the filter under the tap. The good bacteria live in the filter sponge. Chlorine in tap water kills them. Rinse the sponge in a bucket of old tank water instead.
- Skipping changes because the water looks clear. Nitrate is invisible. Clear water can still be dirty water.
Questions and answers
How often should I change the water in a 10 gallon tank?
Change about a quarter of it, 2 to 2.5 gallons, once a week. If the tank is crowded, change a third.
Can I change too much water?
Changing up to half is safe if the new water is the same temperature and has been treated with conditioner. Avoid changing all of it at once, which shocks the fish.
Do I need to take the fish out for a water change?
No. Leave the fish in the tank. Catching them is far more stressful than a careful water change.
Do I still need water changes if I have a good filter?
Yes. A filter turns harmful waste into nitrate, but it cannot remove the nitrate. Only changing water does that.
How long should tap water sit before adding it to the tank?
With a water conditioner, it does not need to sit at all: the conditioner works in seconds. Letting water stand overnight no longer works in many places, because water companies now use chloramine, which does not evaporate.