In short
Most of the time the filter just needs priming or the tank needs topping up. If it still won't pump, check that the cartridge is seated the right way and that the impeller turns freely. Marineland says a small piece of carbon or gravel can jam the impeller. Unplug the filter before you reach into it.
1. Unplug it, then plug it back in
Marineland's Penguin manual starts here when the filter isn't pumping. If the motor hums but nothing moves, go to step 5.
2. Check the water level
A hang-on-back filter pulls water up its intake tube, so the tank has to be full enough. Marineland says to keep the water about 1/2 inch below the frame on Penguin filters. Aqueon's QuietFlow PRO manual says to fill to about 1 inch from the top edge, then top off to the bottom edge of the frame.
3. Prime the filter box
Pour aquarium water into the filter box until it's full, then plug it in. This gives the intake something to pull against. Some filters need this every time the box drains during a cartridge change.
4. Seat the cartridge the right way
A cartridge that's backward or sitting high lets water go around it or blocks the flow path. Aqueon says to fit its cartridges with the FRONT tab toward the front of the filter. Push the cartridge all the way down into its slot.
5. Free the impeller
Marineland says to make sure the impeller housing is seated and that the impeller turns freely, because a carbon or gravel particle can block it. With the filter unplugged, pull the intake tube, take out the impeller, rinse it and its well, and put it back. Aqueon's internal filter instructions include the same impeller cleaning step.
6. Check the intake tube
Brush out the intake tube and its strainer. If the filter keeps sucking air and won't settle, Marineland says to check the intake tube for cracks.
Don't run it dry: if the motor runs with no water moving, unplug it while you work through the checks.
Cloudy water after the change?
Aqueon says to rinse a new cartridge under cold water before fitting it. An unrinsed cartridge can release loose carbon dust. A new Bio-Wheel or bio media also needs time: Marineland's manual gives 24 to 48 hours for the Bio-Wheel to start turning and break in.
Still not working?
If the impeller is clean and spins by hand, the tank is full and the filter is primed but the motor is silent, the motor may have failed. Check the warranty on the box before you buy a new filter. And if the cartridge just doesn't seem to fit, double-check the size in the cartridge size chart.
Questions people ask
Why did my aquarium filter stop after I changed the cartridge?
The usual causes in maker manuals are a filter that needs priming, a water level that's too low, an impeller that's out of place or jammed, or a cartridge put in backward. Marineland says a bit of carbon or gravel can block the impeller.
How full should the tank be for a hang-on-back filter?
Marineland says to keep the water about 1/2 inch below the frame for Penguin filters. Aqueon's QuietFlow PRO manual says to fill to about 1 inch from the top edge, then top off to the bottom edge of the frame.
Which way does an Aqueon cartridge go in?
Aqueon says to put it in with the tab marked FRONT facing the front of the filter.
Why is the water cloudy after a new cartridge?
Aqueon says to rinse a new cartridge under cold water before you fit it. Loose carbon dust from an unrinsed cartridge is a common cause of a cloudy tank right after a change.
Sources
Last checked Oct 25, 2026. Fit lists and change intervals come from Tetra, Aqueon and Marineland product pages and filter manuals. Unplug the filter before you reach into it.
