How Often Should You Change Aquarium Water?
3 minute read · Updated November 4, 2026
Edited by Ivan F.
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Quick answer
Most sources give a partial change every one to four weeks, and the right amount depends on your stocking and your test readings. Never change all of it. Treat new water and match its temperature. Always remove old water rather than only topping up, because topping up alone can lead to a dangerous pH crash.
What the sources say
There is no single answer, because tanks differ. This is what we found:
| Source | How often | How much |
|---|---|---|
| A veterinary manual, general | Every 2 weeks | Depends on stocking |
| A veterinary hospital group | Every 2 to 4 weeks | Partial |
| A veterinarian, bettas | Weekly | 15% to 25% |
| A veterinarian, goldfish | Every 2 to 4 weeks | 10% to 25% at most |
| A veterinary manual, very small tanks | More often | Perhaps as much as 50% a week |
Our advice for a beginner: start with the veterinary manual’s 2 weeks, and adjust from your readings. If nitrate climbs quickly, or you have a small tank or many fish, change more often.
Let your test kit decide
A schedule is a starting point. Your readings show whether it works. See what the readings mean.
- Ammonia or nitrite showing: a problem, and not routine. A fish veterinarian says to change water straight away if ammonia is above 0.1 mg/L, nitrite is above 0, or nitrate is above 20 mg/L. See our cycling guide.
- Nitrate climbing: your schedule may be too thin.
- Tank and tap pH far apart: that veterinarian says to make smaller, gradual changes if they differ by more than 0.5.
How to do it
Prepare the new water
Treat it with a conditioner, and match the temperature to the tank. A veterinarian writing for PetMD says new water should be conditioned and at the same temperature.Unplug the heater first
Do this before the water level falls. Our heater guide explains why. Unplug the filter too if the water will drop below it.Siphon from the bottom
A veterinary manual suggests a vacuum that pulls out solid waste from the gravel as the water comes out, so you clean and change in one job.Rinse filter media in the old water
If the filter needs it, use the water you just removed. Never rinse media under the tap.Refill gently, then plug back in
Plug in only once the water is back above the heater.
What to avoid
- Changing all of it. A retailer’s expert says never to change 100%, and a veterinarian says replacing it all removes the helpful bacteria.
- More than half at once. A fish veterinarian says not to replace more than 50% at one time when treating a new tank problem.
- Only topping up. See below.
Frequently asked questions
How much water should I change?
Sources differ, and they do not all mean the same tank. Examples run from 10% to 50%, and a fish veterinarian says not to replace more than 50% at once when treating new tank problems. Let your test readings guide you.
Can I just top up the water that evaporates?
Not on its own. A veterinary manual explains that adding water without removing old water lets acids build up. That can end in a pH crash called old tank syndrome. Topping up is fine as well as water changes, not instead of them.
Do I need to remove my fish to change the water?
No source we read asks for it in a routine change. Draining and replacing all the water is what to avoid, because it removes helpful bacteria. Change part of it with the fish in the tank.
Do small tanks need more water changes?
Often yes. A veterinary manual says very small tanks may need as much as 50% changed each week.