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.
For most community tanks, change about 25% of the water once a week, always replacing it with dechlorinated, temperature-matched water. That's a starting point, not a law — the real schedule is set by your nitrate reading. Test the water, and when nitrate creeps up, change more or more often. Vacuum the gravel as you drain, never change all the water at once, and don't deep-clean the filter on the same day.
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.
Common questions
How much water should I change and how often?
About 25% once a week suits most community tanks, always with dechlorinated, temperature-matched water. Treat that as a starting point and adjust using your nitrate reading rather than the calendar.
Can you change too much water at once?
Large sudden changes can shift temperature and chemistry faster than fish like, so routine changes are usually kept to 25 to 50%. Bigger changes are fine in an emergency, matched carefully for temperature and treated for chlorine.
What sets the real water-change schedule?
Your nitrate reading. Let nitrate sit around 20 to 40 ppm between changes and adjust frequency or volume to keep it there. A heavily stocked tank needs more; a lightly stocked, planted one needs less.