Why is my aquarium filter running but not moving water?

The most common causes are a clogged intake or filter media, trapped air, a kinked hose, low aquarium water, or a jammed impeller. Unplug the filter, clear the water path, refill or prime the housing, and inspect the impeller before assuming the motor has failed.

How do I restart a hang-on-back filter that stopped flowing?

Unplug it, confirm the intake is submerged and clear, then fill the filter box with aquarium water if the manual requires manual priming. Check for a clogged cartridge and debris around the impeller. Restart it only after the pump chamber is filled.

Can dirty filter media stop aquarium water flow?

Yes. Sludge-packed sponges, filter floss, and cartridges can restrict the pump's intake or outlet. Rinse reusable media gently in removed aquarium water and replace disposable media that remains blocked. Avoid replacing all biological media at the same time.

Why does my sponge filter have bubbles but little water flow?

The air pump may be too weak, the airline may leak, the check valve may be installed incorrectly, or the sponge and airlift tube may be clogged. Clean the sponge with removed aquarium water and inspect the tubing and airlift tube for blockages.

How long can fish go without a working filter?

There is no single safe time because it depends on stocking, temperature, tank size, and surface agitation. Restore aeration and water movement as soon as possible, monitor fish behavior, and test ammonia and nitrite if the interruption lasts or biological media dries out.

Aquarium Filter Not Flowing: Causes and Fixes

An aquarium filter that stops flowing is usually dealing with clogged media, a blocked intake or hose, trapped air, low water, or an incorrectly seated impeller. Unplug the filter before opening it, then work through these checks in order.

If your aquarium filter is not flowing, unplug it and check the filter media, intake, hoses, water level, and impeller in that order. Most flow problems are caused by a blockage, air trapped in the housing, a loose seal, or media packed too tightly—not a failed motor. Keep water moving with a separate air pump or temporary circulation method if the tank is heavily stocked or fish are gasping at the surface.

1. Unplug the filter and check the filter media

Disconnect power before removing the lid, intake, or impeller cover. Water and electricity are a dangerous combination, so keep the plug and outlet dry.

Remove the filter media and look for:

Brown sludge or decomposing debris
A clogged intake sponge or prefilter
Filter floss packed flat with waste
Carbon or other media swollen inside a cartridge
Media loaded into the wrong compartment
Media packed so tightly that water cannot pass through it

If the filter runs normally with the media removed, the media is restricting flow. Rinse reusable biological media and sponges gently in a container of removed aquarium water. Do not rinse beneficial biological media under untreated tap water unless the manufacturer specifically says it is safe. Replace disposable floss or damaged cartridges when cleaning no longer restores flow.

Do not replace all established biological media at once unless it is contaminated or unusable. Removing all of it can reduce the tank's biological filtration.

Check whether the media is installed correctly

Some hang-on-back and canister filters require media to sit in a particular order or direction. Follow the filter's manual. A cartridge installed backward, a missing divider, or an overfilled basket can force water around the media or stop circulation altogether.

2. Inspect the intake, hose, and outlet

A filter cannot move water if its intake is blocked. Check the entire water path:

Remove plant leaves, gravel, snails, fish food, and debris from the intake strainer.
Clean the intake sponge or prefilter.
Look through the hose for sludge, algae, or a collapsed section.
Check for sharp bends or kinks.
Confirm the outlet is not blocked by a spray bar, nozzle, or accumulated debris.
Make sure the intake tube is firmly connected.

For flexible tubing, disconnect it only after the filter is unplugged and place a towel under the connection. A long, narrow, or heavily fouled hose can reduce flow even when the pump motor is running.

3. Check for air locks and trapped air

Air in the pump chamber can prevent a filter from drawing water. This is common after cleaning, refilling, moving a canister filter, or allowing the aquarium water level to fall.

Hang-on-back filters

Make sure the filter box is filled with aquarium water before switching it on. Some models self-prime, while others require manual filling. If the motor hums but no water moves, turn it off, refill the housing according to the manual, and restart it.

Never let a hang-on-back filter run dry for an extended period. The impeller and motor assembly may overheat or wear prematurely.

Canister filters

Confirm that the canister is below the aquarium if the manufacturer requires that arrangement. Check that the hoses are connected to the correct inlet and outlet ports, then prime the canister using its primer or the procedure in the manual. Do not repeatedly run a dry canister pump.

Sponge filters

A sponge filter does not pump water by itself. It needs an air pump, airline tubing, and an airlift tube. If bubbles are present but water is barely moving, check the air pump, tubing, check valve, and airlift tube for blockages or leaks.

4. Look for leaks and poorly seated seals

A small air leak on the intake side can stop a filter from priming or cause weak, intermittent flow. Inspect:

O-rings and gaskets
Hose connections
Cracked intake tubes
Loose filter lids
Canister clamps
Drain or quick-disconnect valves
Split airline tubing on sponge filters

Re-seat the lid and gasket carefully. Clean debris from the sealing surfaces. Replace a swollen, cracked, flattened, or missing gasket with the correct part for the filter. Do not force a lid or clamp closed if the gasket is out of position.

Only use lubricant if the manufacturer recommends it and specifies the correct type. An unsuitable lubricant can damage seals or contaminate aquarium water.

5. Confirm the water level and filter setup

Low water can expose the intake, create a vortex, or prevent a hang-on-back filter from maintaining its siphon. Fill the aquarium to the operating level specified by the filter manufacturer, while leaving room for displacement and possible splash-over.

Also check that:

The filter is sitting level and securely on the tank rim.
The intake is fully submerged.
The outlet is not positioned so high that it loses its siphon.
A spray bar is installed with the correct end cap and orientation.
The filter is rated for the aquarium's actual setup, including substrate, decorations, and densely packed media.

A filter's advertised flow rate is usually measured under favorable conditions. Flow can be lower once hoses, media, lift height, and aquarium plumbing are added. If the filter has always produced weak flow, it may be undersized for the tank or the selected media arrangement may be too restrictive.

6. Check whether the drum, basket, or bag is full

A canister, internal filter, or pond-style aquarium filter may stop flowing when its collection area is packed with debris. Empty the debris compartment and clean the mechanical prefilter. Check the manual before rinsing or replacing biological media.

If the filter uses a filter bag or disposable cartridge, replace it when it is saturated and cannot be cleaned effectively. A full bag can cause bypass, overflow, or a sharp drop in return flow.

7. Inspect the impeller and impeller well

If the filter still does not flow after the intake and media are clear, unplug it and inspect the impeller. Remove the impeller cover as described in the manual and look for:

Gravel or plant fragments lodged inside
Hair or string wrapped around the impeller
Slime or mineral deposits on the impeller or shaft
A cracked magnet or damaged blades
A bent or broken ceramic shaft
A worn rubber bushing

Clean the impeller well and parts with aquarium-safe methods recommended by the manufacturer. Do not use force on a stuck ceramic shaft. If the impeller is cracked, noisy, wobbly, or missing parts, replace the exact compatible impeller assembly rather than modifying it.

A motor that hums without turning the impeller often indicates a jammed or worn impeller. A motor that runs but produces very little water may have a damaged impeller, an air lock, or a serious internal blockage.

8. Treat electrical symptoms as a separate problem

Stop using the filter and disconnect it if you notice a burning smell, melted plastic, sparking, repeated breaker trips, a damaged cord, water entering the electrical housing, or unusual electrical noise. Do not open or repair a sealed motor housing unless the manufacturer specifically provides a safe service procedure.

Check that the outlet or power strip works with another safe device, but do not bypass a ground-fault circuit interrupter or continue resetting a tripped safety device. For an electrical fault, contact the manufacturer or a qualified professional. Replacement is usually more sensible than repairing a small aquarium filter with a damaged sealed motor.

What to do if fish are losing oxygen

If fish are gasping, gathering at the surface, or showing unusual lethargy, restore water movement promptly. Use a properly sized air pump and air stone, lower the water temperature only if appropriate for the species, and increase surface agitation without creating dangerous current. Avoid adding untreated tap water or medications as a substitute for diagnosing the filter.

Test ammonia and nitrite if the filter has been stopped for a prolonged period or if biological media was replaced. Follow an aquarium maintenance plan appropriate for the tank and its inhabitants. If fish are in distress, contact an aquatic veterinarian or experienced aquarium professional.

When to replace a part or the filter

Replace a compatible filter cartridge, sponge, hose, gasket, intake strainer, or impeller when that part is damaged or cleaning no longer restores normal flow. A replacement impeller is often the reasonable fix when the motor housing is sound but the impeller is cracked or worn.

Replace the entire filter when the motor housing leaks, the electrical cord is damaged, the housing is cracked, compatible replacement parts are unavailable, or the unit repeatedly loses flow after thorough cleaning and correct priming. Choose a replacement based on the tank's volume, livestock load, required lift height, media capacity, and acceptable noise level—not only the manufacturer's maximum flow figure.