How Often Should You Do a Water Change?
How often should you change aquarium water? About 25% weekly for most tanks — but let your nitrate reading, not the calendar, set the real schedule.
Water changes are the single most important thing you do for a tank, and the one beginners most often get wrong — too little, too much, or with the wrong water. Here is how to do them right.
How much, how often
| Tank | Starting point |
|---|---|
| Typical community tank | ~25% once a week |
| Heavily stocked (goldfish, cichlids) | 25–50% weekly, sometimes more |
| Lightly stocked or planted | 10–20% every one to two weeks |
These are starting points, not rules. The honest schedule is set by one number: nitrate. Test it, and if it climbs toward 40 ppm, change more water or change more often. If it stays low, you can ease off. The calendar is a guess; the test is the answer.
How to do a water change
- Test first (at least occasionally) so you know where nitrate sits.
- Vacuum the gravel with a siphon as you drain, pulling out debris and uneaten food.
- Drain about a quarter of the water — no need to remove the fish.
- Refill with fresh water that is dechlorinated and matched to the tank temperature (within a couple of degrees).
- That's it — turn any equipment you paused back on.
The rules that keep it safe
- Always dechlorinate. Tap water's chlorine and chloramine kill the good bacteria and harm fish — a water conditioner neutralises it in seconds.
- Match the temperature. Dumping in cold water shocks fish; get it close before it goes in.
- Never change all of it. A giant change swings the water chemistry and stresses fish more than the dirty water did. Little and often beats big and rare.
- Don't deep-clean the filter the same day. Your filter holds most of the tank's beneficial bacteria — rinse it (in old tank water, never tap) on a different day so you don't disturb everything at once.
Let your test kit set the schedule
The whole point of a water change is to keep waste — measured as nitrate — from building up. So the test kit isn't optional: it's what turns "change some water sometimes" into a real routine. Rising nitrate means change more; steady low nitrate means you've found your rhythm. The numbers to watch are covered in our water-parameters guide.
Sources
- Aquarium water changes — Aquarium Science, on frequency and nitrate.
- r/Aquariums beginner wiki — community consensus on routine changes.
A weekly 25% change with dechlorinated, temperature-matched water keeps most community tanks healthy — but treat that as a default, not a rule. Let your nitrate test set the real pace: more fish and more food means more waste and more frequent changes. Vacuum the gravel, never swap all the water at once, and leave the filter alone the same day.
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