How often should you change aquarium water?
AquaLog guide · Published
In short: For most stocked freshwater aquariums, change 10–25% of the water once a week. Let your nitrate readings fine-tune it: keep nitrate below about 20–40 ppm. Always treat new water with dechlorinator and match its temperature to the tank.
Why change water at all?
The filter turns toxic ammonia and nitrite into nitrate, but nothing in a typical aquarium removes nitrate fast enough. It builds up week after week, along with dissolved waste that yellows the water and feeds algae. Changing part of the water dilutes all of it and replaces minerals the fish and plants use up.
How much and how often?
| Tank | A good starting schedule |
|---|---|
| Lightly stocked, well planted | 10–15% weekly, or 20–30% every two weeks |
| Typical community tank | 20–25% weekly |
| Heavily stocked, or messy fish such as fancy goldfish | 25–50% weekly, sometimes more |
| Nano tanks under about 40 L | Small weekly changes; small volumes swing quickly |
How do you know your schedule is right?
Test nitrate just before each water change. If it has climbed above your target since the last change, change more water, change it more often, feed less or keep fewer fish. If it stays low, your schedule is working. Many keepers aim for under 20 ppm; most freshwater fish tolerate up to about 40 ppm, and sensitive species do better at the low end.
How do you change water safely?
- Switch off the heater, and the filter if the water will drop below its intake. A hot heater out of the water can crack.
- Siphon water out with a gravel vacuum, cleaning the substrate as you go.
- Prepare the new water: match the tank temperature within about 1–2 °C and add dechlorinator as the label directs.
- Refill slowly so you don't blast the substrate or stress the fish.
- Switch everything back on and check the heater is working.
What are the common mistakes?
- A huge change after weeks of neglect. When nitrate is very high, the sudden shift in water chemistry can shock fish. Do several smaller changes over a few days instead.
- Cleaning the filter on the same day. Spread big jobs out so the bacteria aren't disturbed all at once.
- Forgetting dechlorinator. Chlorine and chloramine harm fish and kill filter bacteria.
- Cold water. A big temperature drop stresses fish, especially warm-water species such as neon tetras.
What about marine tanks?
Use saltwater mixed ahead of time, matched to the tank's salinity and temperature. Top up evaporation with fresh RO water, not saltwater, because only the water evaporates and the salt stays behind.
AquaLog logs each water change in two taps and reminds you when the next one is due, so you can see your real schedule instead of guessing. Start your log, or read the maintenance checklist.