The short answer
Priming means filling the canister and hoses with water so the pump doesn’t try to run on air. Most modern canisters have a primer button or lever — pump it a few times to draw water in. If yours doesn’t, you fill the canister manually before sealing it, or use the intake tap to siphon water through. Once water is flowing, plug in and the filter self-primes the rest of the way.
Method 1 — the primer button
With the filter below the tank and both taps (inlet and outlet) open:
- Push the primer button repeatedly in quick, firm strokes.
- You’ll hear water rushing into the canister and see the hoses fill.
- Keep going until the canister is full and bubbles stop.
- Plug the filter in — flow should start within seconds.
Method 2 — fill it manually
No primer? Before you seal the lid, pour dechlorinated water directly into the canister until it’s full. Reseal, open both taps, and the head of water in the tank plus the full canister is usually enough to start a siphon when you power on.
If it won’t hold the prime
Air trapped in the system is the usual culprit. Make sure both taps are fully open, the hoses have no high loops trapping air, and the lid gasket is seated and lightly greased. Gently rocking the canister helps burp out bubbles.
If it still won’t start, see why is my canister filter not priming for the full checklist. New to canisters? Our best external aquarium filter picks and the aquarium filters hub will help you choose, and how to set up a new aquarium filter covers the whole install.