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Fix an aquarium filter that will not filter

We'll confirm the symptom, rule out power and priming, then check the intake tube, media, and impeller—or tell you when to call a pro.

Troubleshooting · Home appliances
15–30 min
What you'll need
  • Bucket (for tank water when rinsing media)
  • Replacement filter media or impeller (if clogged beyond cleaning or damaged)
Step 1 of 12

Quick triage — pick your path

Quick triage — pick your path

Choose the option that matches what you see. You can jump straight to that section.

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Steps

Goal: Confirm the symptom, rule out power and priming, then check the intake tube, media, and impeller.

  • Check the outlet for water flow. No flow, weak flow, or cloudy water that does not clear means the filter is not filtering.
  • Good: No flow or weak flow—proceed to Power and priming.
  • Bad: Strong flow and clear water—filter is working; if water quality is poor, check water chemistry or overfeeding.

Power and priming

Goal: Rule out power loss and air lock before opening the filter.

  • Confirm the filter is plugged in and the circuit breaker has not tripped. Reset the breaker if tripped.
  • If the filter is a hang-on-back or canister, prime it. Hang-on-back: fill the pump chamber with tank water until it overflows into the tank, then plug in. Canister: fill the canister with tank water, reconnect hoses, open inlet and outlet valves, then plug in.
  • Good: Water flows after priming. The filter is filtering.
  • Bad: Still no flow—proceed to Intake and media.

Intake and media

Goal: Check the intake tube and filter media for blockages.

  • Confirm the intake tube is fully submerged and not blocked by gravel, plants, or debris. Pull the intake out and inspect the strainer. Rinse in tank water.
  • Remove the media (sponge, cartridge, or other) and rinse it in a bucket of tank water. Do not rinse in tap water—chlorine kills beneficial bacteria. Replace if falling apart or heavily clogged.
  • Good: Intake clear and media cleaned or replaced. Flow should improve.
  • Bad: Flow still weak—proceed to Impeller path.

Impeller path

Goal: Inspect and clean or replace the impeller.

  • Unplug the filter. Remove the impeller cover per your model—often a twist-off or screw. Pull out the impeller and inspect for debris, hair, or damage.
  • Rinse the impeller and clear the impeller shaft in tank water. Reinstall and prime if needed.
  • If the impeller is cracked, has broken blades, or does not spin after cleaning, replace it with an exact match. Search “[brand] [model] impeller” for the part.
  • Good: Impeller spins and flow is strong. The filter is filtering.
  • Bad: Motor does not run or flow stays weak after cleaning—call a pro.

When to get help

Call an aquarium or appliance technician if:

  • You have cleaned the impeller and intake, replaced media, and the motor still does not run or flow rate stays weak.
  • The filter leaks at seals or fittings—do not run it; unplug and call a pro.
  • You are not comfortable disassembling the unit.

Verification

  • Water flows from the outlet at a strong flow rate.
  • Cloudy water clears over time as the filter runs.
  • No humming without flow (impeller spinning).
  • No leaks at the filter body, hoses, or fittings.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm symptom Verify no flow, weak flow, or water stays cloudy.
  2. Power and priming Check power and prime the filter if hang-on-back or canister.
  3. Intake and media Check intake tube is submerged and clear; clean or replace media.
  4. Impeller Inspect and clean impeller; replace if damaged.
  5. Call a pro Motor does not run after cleaning, flow stays weak, or filter leaks—call a technician.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Filter type (hang-on-back, canister, or internal)
  • Whether the impeller spins
  • Whether the intake is submerged and clear
  • Media condition (clogged, replaced recently)
  • Steps already tried

Does the filter have no flow or weak flow?

Check the outlet. No water or a trickle means the filter is not filtering. Cloudy water that does not clear can mean clogged media or weak flow.

Check the outlet for water flow. Good: no flow, weak flow, or cloudy water that does not clear—filter fault. Bad: strong flow and clear water—filter is working; if water quality is poor, check water chemistry or overfeeding.

Yes No

You can change your answer later.

Filter is working

Strong flow and clear water—the filter is working. If water quality is poor, check water chemistry, overfeeding, or tank maintenance.

Done

Is power on and the filter plugged in?

Circuit breaker and plug connection can stop the filter. Check both.

Confirm the filter is plugged in and the circuit breaker has not tripped. Reset the breaker if tripped. Good: power on, filter responds. Bad: breaker keeps tripping—unplug and call a pro; do not keep resetting.

Yes No

You can change your answer later.

Did priming restore flow?

Hang-on-back and canister filters can air-lock. Prime first: hang-on-back—fill pump chamber with tank water until it overflows, then plug in; canister—fill canister with tank water, reconnect hoses, open valves, then plug in. Internal filters rarely need priming—skip to intake.

If hang-on-back or canister: prime the filter. Hang-on-back: fill the pump chamber with tank water until it overflows, then plug in. Canister: fill the canister with tank water, reconnect hoses, open valves, then plug in. Good: water flows after priming. Bad: still no flow or internal filter—proceed to intake and impeller.

Yes No

You can change your answer later.

Flow restored

The filter is now filtering. Confirm flow rate is strong and water clears over time.

Done

Is the intake tube submerged and clear?

The intake must be in the water and free of debris. A blocked or raised intake stops flow.

Confirm the intake tube reaches into the water. Pull it out and inspect the strainer. Rinse in tank water. Good: intake submerged and clear—proceed to impeller. Bad: intake blocked or above water—fix and retest.

Yes No

You can change your answer later.

Clear intake and retest

Submerge the intake fully. Rinse the strainer in tank water. Reinstall and prime if needed. Test flow. If still no flow, proceed to impeller.

Done

Does the impeller spin when the filter runs?

The impeller moves water. Debris or damage can stop it.

Unplug the filter. Remove the impeller cover. Pull out the impeller and inspect for debris or damage. Clean the shaft and blades. Reinstall and prime. Spins: filter should flow—check media if flow is weak. Does not spin or damaged: replace impeller or call a pro.

Yes No

You can change your answer later.

Is the media clogged?

Clogged media reduces flow rate. Rinse in tank water or replace.

Remove the media. Rinse in a bucket of tank water (not tap). Replace if falling apart or heavily clogged. Reinstall and test. Good: flow improves. Bad: flow still weak or motor does not run—call a pro.

Yes No

You can change your answer later.

Clean or replace media and retest

Rinse media in tank water. Replace if needed. Reinstall and test. Flow should improve. If not, call a pro.

Done

Replace impeller and retest

Replace the impeller with an exact match. Search "[brand] [model] impeller" for the part. Install, reassemble, and prime. The filter should flow. If it still does not, call a pro.

Done

Call a technician

Call an aquarium or appliance technician if: the motor does not run after cleaning the impeller; flow stays weak after cleaning intake and media; the filter leaks; or you are not comfortable disassembling the unit. Do not run a filter that leaks.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would an aquarium filter not filter?
Common causes: air lock (needs priming), clogged intake tube or media, debris in the impeller, or motor failure. Power loss or a tripped breaker can also stop the filter. Check power and priming first, then intake, media, and impeller.
How do I prime an aquarium filter?
Hang-on-back: fill the pump chamber with tank water until it overflows into the tank, then plug in. Canister: fill the canister with tank water, reconnect hoses, open valves, then plug in. Internal: submerge fully, shake to release air, then plug in. The pump must move water, not air.
When should I call a technician for an aquarium filter that will not filter?
Call an aquarium or appliance technician if you have cleaned the impeller and intake, replaced media, and the motor still does not run or flow rate stays weak; if the filter leaks; or if you are not comfortable disassembling the unit.

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