If an aquarium filter stops pumping, first verify power and water level, then unplug it before inspecting the intake, mechanical media and impeller. HOB and canister filters often need to be re-primed after maintenance or power loss. Air locks, clogged prefilters, kinked hoses and a jammed impeller are common causes. Keep biological media wet while troubleshooting and add aeration if the outage is prolonged.
Do not let a silent filter sit unnoticed. Even if the fish look fine, oxygenation and biological filtration may already be reduced.
Check the simplest things first
- Power is available and the plug or breaker is working.
- The intake is below the water line.
- The tank water level is high enough for the design.
- Valves are open and hoses are not kinked.
- The filter is correctly assembled after cleaning.
Prime the filter
HOB and canister filters may not self-start if the pump contains air. Fill or prime the unit exactly as the manufacturer specifies.
Never run an impeller dry for long periods.
Inspect the intake and media
A clogged prefilter, intake strainer or dense floss can reduce flow to almost nothing. Clean the mechanical stage first while preserving biological media.
If flow returns, the motor was probably not the problem.
Inspect the impeller safely
Unplug the unit, remove the impeller and check for hair, sand, snail shells or mineral deposits. Refit the shaft and cover correctly.
A cracked or swollen impeller may need replacement.
Protect the aquarium during downtime
Add aeration, reduce feeding and keep biological media wet. After a long outage, test ammonia and nitrite when the filter is restored.
Frequently asked questions
Why does my filter hum but not pump?
The motor may be energised while the impeller is jammed or the pump is air-locked.
Can I leave the filter off until tomorrow?
Avoid unnecessary downtime. Restore circulation or provide aeration as soon as possible.
Should I replace all the filter media after a blockage?
No. Clean reusable media as appropriate and preserve the established biological colony.
Sources and further reading
This guide separates general aquarium arithmetic from fish-welfare decisions. For species-specific limits, always check the needs of the fish you intend to keep.