How often should you change fish tank water? How much, and how to do it

How often: part of the water every 1–2 weeks

Most guidance says to change part of the water every 1–2 weeks, not to empty the tank now and then. Here’s how a few reliable sources put it:

Source	How often	How much
RSPCA (UK)	Every week	30%
Merck Veterinary Manual (written by University of Florida veterinarians)	Every 2 weeks	Depends on how heavily the tank is stocked
VCA Animal Hospitals (Gregory Rich, DVM)	Every 2–4 weeks	Not specified

Why change water if you have a filter? The RSPCA explains it this way: fish release ammonia, the filter’s bacteria convert it to nitrite and then to the safer nitrate — but nitrate still causes problems at high levels, and only water changes dilute it.

Small tank, lots of fish or generous feeding: start weekly. Big tank, few fish: every 2 weeks can work. The RSPCA notes that larger volumes keep temperature and water conditions more stable, so smaller tanks shift faster and can’t go as long between changes.

How much: no single percentage fits every tank

Use the RSPCA’s 30% a week as a starting point, then adjust by testing. The Merck Veterinary Manual doesn’t give a fixed percentage — it says the amount depends on stocking — and it recommends testing pH, alkalinity, ammonia, nitrite and nitrate every 2 weeks, which fits neatly on water-change day.

The RSPCA gives the safe level for both ammonia and nitrite as zero. If tests show more than that, it recommends changing half the water every day and retesting until conditions improve.

How to do a water change

1. Leave the fish in: the RSPCA’s method is to keep the fish in the tank and siphon out part of the water.
2. Clean the gravel as you go: the Merck Veterinary Manual says cleaning should include stirring up the gravel to release trapped waste, which a gravel vacuum can remove during the water change.
3. Rinse filter media in tank water: the RSPCA says to swish the filter sponge or media in the water you just removed, and never under the tap, which kills the beneficial bacteria. Clean the impeller too.
4. Dechlorinate the new water: the CDC says fish shouldn’t be kept in water containing chlorine or chloramine, because they absorb water directly into their bloodstream and even low levels can make them sick. A water conditioner is the simplest fix. Standing water loses chlorine — the RSPCA says 24 hours, the CDC says a few days — but chloramine doesn’t leave that way, so if your water supplier uses chloramine, use a conditioner.
5. Match the temperature: the RSPCA warns that new water at a different temperature can shock the fish, and suggests a separate heater to warm it first.

If your filter takes replaceable cartridges, the Merck Veterinary Manual says to change them about monthly, sooner if they clog, and to replace filter carbon every 1–3 months.

Cloudy after a water change? Don’t judge by looks

You can’t tell water quality by looking — use test kits. The RSPCA reminds owners that clear water doesn’t always mean healthy water, and recommends testing every two weeks, with ammonia and nitrite kits as essentials. Cloudiness can come from more than one thing, such as debris stirred up from the gravel, so test before deciding whether to change more water.

A new tank needs patience: the VCA veterinarian recommends running the water through the filter for 4–6 weeks before adding fish, so the system can settle.

Keep it hygienic

Wash your hands with soap and running water before and after working on the tank. The CDC also recommends:

• Wearing gloves when cleaning the aquarium, especially if you have cuts on your hands.
• Dumping tank water in a bathtub, laundry sink or outside rather than the kitchen sink when possible, then cleaning and disinfecting the sink.
• Not letting children under 5 clean or handle aquarium items; people with weakened immune systems should let someone else care for the tank when possible.

Common mistakes

• Emptying the whole tank: the guidance is about changing part of the water. Scrubbing everything, filter media included, under the tap also washes away the bacteria that break down waste.
• Pouring in untreated tap water: chlorine harms both fish and filter bacteria.
• Only topping up evaporation: the Merck Veterinary Manual lists topping up as a weekly job and water changes as a separate one every 2 weeks — one doesn’t replace the other.
• Trusting clear water: clear doesn’t mean free of ammonia and nitrite.
• A big temperature gap: cold or warm new water can shock the fish.

How to remember

Pick one day a week for water changes, like Saturday morning. The trouble starts when you skip one: after that it’s hard to say which week you last did it, and when someone else helps with the fish, it’s easy to change it twice or not at all.

Track “aquarium water change” as a weekly item in a shared list, and note how much you changed and whether you vacuumed the gravel or rinsed the filter, so you know what’s due next time. Filter cartridges and carbon run on a longer cycle, so give them their own item. If you also have a cat, the pet water fountain needs cleaning every week or two as well, so both can go on the same day — and the water filter jug in the kitchen is another filter that goes by the calendar.