For an established freshwater aquarium, plan a partial clean every week. Remove waste from the substrate and replace part of the water, then adjust the amount and frequency according to water-test results, fish load, tank size and filtration. Published guidance ranges from 10–25% weekly to 30% weekly; PetMD also lists 25% every other week as an option. Cleaning is ongoing maintenance, not an occasional full teardown.
What “cleaning” a fish tank actually includes
Several jobs are often called cleaning, but they do not happen on the same timetable:
- Partial water change: remove some old water and replace it with conditioned water at the aquarium’s temperature.
- Substrate cleaning: siphon waste and uneaten food from gravel or sand while removing the old water.
- Algae removal: wipe or scrape the glass when growth obstructs the view or affects equipment. Use a tool compatible with the tank material.
- Filter care: rinse clogged media gently in water removed from the aquarium so beneficial bacteria are not destroyed.
Do not routinely empty and sterilize the entire aquarium. Large, sudden changes can destabilize temperature, chemistry and biological filtration.
How often to change water in an established tank
There is no single percentage that suits every aquarium. These are the published starting points:
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| Source | Starting recommendation | How to use it |
|---|---|---|
| PetMD (Jessie Sanders, DVM, DABVP; May 7, 2026) | 10% weekly or 25% every other week | Use as a starting schedule for an established tank, then verify with tests. |
| RSPCA Australia | 10–25% weekly after the biofilter matures | Choose a volume within the range based on stocking, filtration and measured water quality. |
| RSPCA UK | 30% weekly | A more conservative weekly schedule for maintaining water quality. |
These recommendations differ because aquariums differ. A heavily stocked or lightly filtered tank may need more frequent changes; a lightly stocked, well-filtered tank may remain stable with the lower end of the range. Test results should decide whether your chosen routine is adequate.
New tanks and new filters need closer attention
During roughly the first four to six weeks, a new aquarium or newly installed filter may not contain enough beneficial bacteria to process toxic waste immediately. RSPCA Australia recommends initially frequent 10–25% partial changes together with gravel cleaning during this establishment period.
Test more often while the biological filter develops, and respond to results rather than waiting for the regular weekly day. Once the biofilter is mature and readings are consistently safe, move to a routine schedule and continue monitoring.
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Use water tests to move the schedule forward
PetMD’s fish-veterinarian-authored guide advises a water change when ammonia is above 0.25 mg/L, nitrite is above 0 mg/L or nitrate is above 20 mg/L. Treat these as PetMD’s guidance, not universal limits for every species or aquarium; species tolerance, pH, temperature and the test method matter. If readings are concerning or fish are unwell, contact a fish veterinarian.
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How to perform a routine weekly clean safely
- Prepare replacement water. Add tap-water conditioner according to its label and bring the water to the aquarium’s temperature. Avoid adding untreated tap water.
- Switch off equipment as needed. Turn off heaters, pumps and filters whenever the water level could expose them or the cleaning process requires it. Follow the equipment maker’s instructions.
- Leave the fish in the aquarium. Catching and relocating them for ordinary maintenance adds handling stress and is unnecessary.
- Siphon the substrate. Use an aquarium gravel siphon or compatible gravel vacuum to lift waste while removing the planned portion of water. Stirring gravel without siphoning leaves waste in the tank.
- Remove algae. Use an aquarium-safe algae scraper suited to glass or acrylic. Do not use soap, household cleaners or tools that can scratch the surface.
- Refill slowly. Add conditioned, temperature-matched water to avoid shocking fish or disturbing the substrate.
- Restart and check equipment. Confirm the filter, heater and circulation are operating normally and that no fish or plants are trapped by equipment.
When and how to clean the filter
RSPCA Australia advises a light rinse when the filter begins to clog or monthly, whichever comes first, while warning against over-cleaning. RSPCA UK likewise says to swish filter media in removed aquarium water, never under the tap. Chlorinated tap water can damage the bacterial community that processes waste.
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Clean only enough to restore flow, retain biological media whenever possible and follow the filter manufacturer’s instructions for cartridges, impellers and replacement intervals. A filter that repeatedly clogs may indicate excess food, too much debris, inadequate mechanical media or a stocking problem rather than a need for increasingly aggressive washing.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What changes the right frequency?
Stocking and feeding
More fish produce more waste. Overfeeding adds uneaten food and can push ammonia and nitrate upward, so a crowded tank or generous feeding routine generally requires closer observation and potentially larger or more frequent partial changes.
Tank size and filtration
Small water volumes can change chemistry quickly. Effective, correctly sized filtration provides biological processing but does not remove the need to siphon waste or replace water. RSPCA UK states: “No fish should be kept in tanks without a well set up and maintained filter.”
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Measured chemistry
Stable readings support your current routine; recurring ammonia, nitrite or high nitrate means the routine, stocking, feeding, filtration or source water needs attention. There is no evidence-based formula that converts tank size and fish count into one exact percentage for every aquarium.
Algae and visible debris
Algae can be removed whenever it becomes objectionable, while substrate waste should be siphoned during regular partial changes. Frequent algae does not by itself justify replacing all the water; investigate light, nutrients and maintenance instead.
Common mistakes to avoid
- Full water replacement as routine: it can cause abrupt temperature and chemistry shifts.
- Washing filter media under the tap: rinse in removed aquarium water instead.
- Using soap or household spray: residues can harm fish and the biofilter.
- Making a sudden, very large change: match temperature and make gradual corrections unless an aquatic veterinarian directs emergency action.
- Removing fish for every cleaning: keep them in place for ordinary maintenance.
- Relying on appearance alone: clear water can still contain toxic ammonia or nitrite, so test it.
A simple schedule you can start today
- Every week: test as appropriate for your system, siphon waste and complete a partial water change using your selected 10–30% starting point; remove algae if needed.
- Weekly while new: during the first four to six weeks or after a new filter is installed, test more frequently and make the RSPCA Australia-recommended 10–25% partial changes as readings require.
- Monthly or when clogged: gently rinse filter media in removed aquarium water, without over-cleaning it.
- Any time tests are abnormal: perform the PetMD-triggered water change guidance for ammonia, nitrite or nitrate, and seek veterinary advice when chemistry or fish health is concerning.
Keep a written log of test results, water volume, filter work and symptoms. After several weeks, that record will show whether your aquarium remains stable between cleanings and whether the interval needs adjustment.
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