Symptoms
- Little or no water flow from the filter outlet
- Rattling, buzzing, or grinding noise from the filter
- Filter runs but the tank stays cloudy
- Filter stopped shortly after being cleaned or restarted
Likely causes
- CommonClogged filter media or intakeDebris, fish waste, and algae buildup in the sponge or cartridge restrict water flow over time.
- CommonImpeller jammed or coated with buildupThe small spinning impeller inside the pump housing can gather grime or catch on debris, reducing or stopping flow.
- CommonAir trapped in the filter (airlock)After cleaning or a water change, trapped air in the housing can prevent the pump from priming properly.
- PossibleImpeller magnet worn or crackedOver months to years, the small magnet that drives the impeller can wear down or crack, weakening flow.
- PossibleIntake tube blocked by gravel, plants, or a snailSomething physically blocking the intake reduces the water reaching the filter at all.
What to do
- Unplug the filter and remove the filter media (sponge, cartridge, or floss) and rinse it in a bucket of removed tank water (not tap water, which can kill beneficial bacteria).
- Take apart the impeller housing per the manual and clean the impeller and its chamber with a soft brush, removing any hair-like buildup.
- Check the impeller for cracks or a worn magnet by comparing its spin resistance to a new one if you have a spare.
- Refill the filter housing with tank water before restarting to help it prime and avoid an airlock.
- Check the intake tube and strainer for blockages from gravel, plant debris, or snails, and clear them.
- Restart the filter and listen for normal operation; a persistent rattle after cleaning suggests impeller wear.
- Clean mechanical filter media roughly every 2–4 weeks to prevent buildup from recurring.
Tools and parts
- Soft brush
- Bucket (for tank water)
Filter impeller assemblySearch: “aquarium filter impeller replacement”
Filter media/cartridgeSearch: “aquarium filter cartridge replacement”
When to call a pro
Replacement impellers are sold for most common filter brands and are a straightforward DIY swap; consider a new filter if the housing itself is cracked.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.