The short answer
A canister that won’t prime almost always has trapped air it can’t clear. The four usual causes are: the filter is placed too high (level with or above the tank), one of the taps isn’t fully open, the lid seal is leaking air, or the impeller is stuck. Work through them in order and flow usually returns in minutes.
The checklist
- Filter too high? The canister must sit below the water line so gravity can start the siphon. Move it into the cabinet if it’s on the floor beside a tall tank stand.
- Taps fully open? Both the inlet and outlet valves must be all the way open. A half-turned tap chokes the flow and traps air.
- Lid seated and greased? A dry or pinched gasket lets air in instead of water. Clean it, apply a smear of silicone grease, and reseat the lid squarely.
- Hoses looped? A high loop in a hose traps an air pocket the pump can’t push past. Route hoses so they run downhill to the canister.
Still nothing?
If air isn’t the issue, check the impeller. Open the pump head, remove the impeller and clear any grit, hair or debris jamming it — a stuck impeller spins but moves no water. Make sure the impeller shaft isn’t cracked.
For the correct start-up steps in the first place, see how do I prime a canister filter. If the noise is the real complaint once it’s running, read how do I keep my aquarium filter quiet. For maintenance, how to clean an aquarium filter and the aquarium filters hub will help.