Most aquariums do well with a 10% to 25% water change once a week. Lightly stocked planted tanks can often stretch to every two weeks, while heavily stocked tanks, goldfish and big cichlids may need 30% to 50% weekly. The right answer for your tank comes from your nitrate readings: change enough water, often enough, to keep nitrate under your target.
How much water should you change? #
| Tank situation | Typical change |
|---|---|
| Lightly stocked, heavily planted freshwater | 10-20% every 1-2 weeks |
| Average community freshwater tank | 15-25% weekly |
| Heavily stocked, goldfish, large cichlids | 30-50% weekly |
| Fish-only saltwater (FOWLR) | 10-20% weekly, or 20-25% every two weeks |
| Reef tank | 10-20% weekly or every two weeks, depending on nutrients and dosing |
| Fish-in cycle in a new tank | As often as needed to keep ammonia and nitrite low |
| Nano tank under 10 gallons | 10-25% weekly, sometimes split into two changes |
Small tanks need attention more often, not less. Waste concentrates faster in a small volume, so one overfeeding mistake shows up in hours rather than days.
How do you use nitrate to set your schedule? #
Nitrate builds up between water changes, and each change removes the same percentage of nitrate as the percentage of water you take out. That lets you work out where your tank will settle.
On a steady routine, nitrate just before each change settles at about:
weekly nitrate increase ÷ fraction of water changed
So if nitrate rises 8 ppm a week:
| Weekly change | Nitrate settles at (just before each change) |
|---|---|
| 10% | About 80 ppm |
| 20% | About 40 ppm |
| 30% | About 27 ppm |
| 40% | About 20 ppm |
| 50% | About 16 ppm |
To find your weekly increase, test nitrate right after a water change and again just before the next one. The difference is what your tank produces in a week. Then pick the change size that keeps the settled level where you want it. For most community tanks that’s under 20 ppm, for the reasons in our guide to a safe nitrate level for a freshwater aquarium. If your tap water already contains nitrate, it sets a floor you can’t change your way below.
A worked example #
Say you keep a 40 gallon community tank and nitrate goes from 10 ppm after Saturday’s change to 22 ppm the next Saturday. That’s 12 ppm a week. At 25% weekly, nitrate settles near 12 ÷ 0.25 = 48 ppm, which is too high. At 40% weekly it settles near 30 ppm, and at 50% near 24. The better fix here is probably to feed less or thin the stocking, then recheck the weekly rise.
Is one big water change better than several small ones? #
For removing nitrate, one big change does more than the same total volume split up. One 50% change removes half the nitrate, while two 25% changes remove about 44%, because the second one works on water that’s already diluted. Big changes also shift temperature, pH and hardness more at once, though. For a stable tank, regular moderate changes are the safer habit. Save big changes for emergencies like an ammonia spike, and match the new water carefully when you do them.
Can you change too much water? #
The new water itself doesn’t hurt fish. The difference between old and new water does. Problems come from:
- Temperature mismatch. Get within a degree or two.
- Chlorine or chloramine. Treat every refill with a dechlorinator, including top-offs.
- A big gap in pH or hardness. If you’ve skipped changes for months, the tank water may have drifted far from your tap water. Get back on schedule with moderate changes a few days apart instead of one large one.
- Salinity mismatch in saltwater tanks. Measure every batch before it goes in.
Do reef and saltwater tanks need a different schedule? #
Saltwater tanks follow the same nitrate logic, with two additions. Water changes put back what corals use up, such as calcium, magnesium and alkalinity, along with trace elements. And each change is a chance to introduce a salinity or temperature swing, so the mixed water needs checking. Reef keepers commonly change 10% to 20% every week or two, and some tanks with dosing systems and light fish loads change less. Our guide to how much salt per gallon covers mixing batches that match your tank.
Does a water change remove beneficial bacteria? #
Not in any meaningful amount. Nitrifying bacteria live on surfaces: filter media, substrate, rock and glass. The water itself holds very few. What hurts them is cleaning filter media in chlorinated tap water or replacing all the media at once.
How do you keep track of water changes? #
It’s easy to lose track of whether the last change was 6 days ago or 12. Writing down the date, the amount, and your nitrate before and after lets you check the math above against what’s actually happening in your tank.
Fish Tank Tracker has a water change log where you enter a percentage or a volume. It shows the calculated volume for your tank and how many days it’s been since the last change. Your nitrate readings chart in the same app, so you can see the sawtooth of weekly build-up and each change’s drop. Add a recurring water change task and you’ll get a notification on the day. All of that is in the free version and stored offline on your phone. For the rest of the routine, see our fish tank maintenance schedule.
Frequently asked questions #
Do you need to dechlorinate water for a water change? #
Yes, if you use tap water. Chlorine and chloramine harm fish gills and kill filter bacteria. If you refill straight from a hose, dose dechlorinator for the full tank volume.
Should I remove fish during a water change? #
No. Netting fish is more stressful than working around them. Just keep the siphon away from small fish and shrimp.
How often should I change the water in a 5 gallon tank? #
Small tanks do best with 20% to 25% weekly, and some keepers split that into two smaller changes a week. Waste builds up faster in less water, so a small tank forgives skipped changes less than a big one does.
Can I skip water changes if my nitrate is low? #
Low nitrate is a good sign, but water changes also replace minerals and remove other waste you don’t test for. Heavily planted, lightly stocked tanks can go longer between changes, but most tanks still benefit from a regular schedule.