Fish tank filter not working: clogged, dried out, or won't prime

If your filter has stopped or slowed to a trickle, get it running again today, but rinse it in tank water first. The helpful bacteria living in the filter need flowing, oxygen-rich water; once the flow stops, they start to die, ammonia starts to build, and a stopped filter that is restarted dirty can pump rotten water straight into the tank.

Most dead filters are not broken. They are clogged, have lost their prime (the water inside them has drained out), or have a jammed impeller. All three are usually fixable in 20 minutes.

How to identify it

Weak flow: the outlet that used to splash now dribbles.
No flow but a humming motor: usually a jammed or broken impeller (the little spinning propeller that moves the water), or air trapped inside.
Rattling, grinding or clicking: a worn or dirty impeller, or a snail shell or gravel stuck in it.
Hang-on-back filter silent, box empty or half-full: it has lost its prime and may be running dry.
Canister gurgling or not moving water after cleaning: trapped air, a hose on the wrong way round, a kinked hose or a closed tap.
A rotten-egg or sewer smell when you open the filter or restart it: bacteria and waste have died and rotted inside.
Cloudy water, fish gasping, or a sudden ammonia reading a day or two after a filter stopped.

When it's a problem

Normal: flow slowly drops over a few weeks as the sponges fill with gunk. That is your reminder to clean them.
Early: flow is half what it was. Clean it this week.
Serious: no flow, or the filter has been off for more than an hour or two. Bacteria start dying from lack of oxygen within roughly an hour or so of the water stopping, depending on how warm and dirty the filter is.
Emergency: the filter has been off for many hours or days, ammonia is showing on a test, or fish are gasping. Treat it as an ammonia emergency.

What to do right now

Unplug the filter. Never work on a plugged-in filter, and don't let a hang-on-back motor keep running dry; it can overheat and burn out.
Add an air stone if you have one, so the fish have oxygen while you work.
Take out a bucket of tank water. This is your rinse water. Never rinse the sponges or bio-media in tap water; chlorine and chloramine kill the bacteria (see chlorine).
If the filter has been off more than a couple of hours, rinse everything before you restart. Squeeze the sponges in the bucket and swish the bio-media. This flushes out the dead, rotting material so it doesn't get pumped into the tank.
Clean the impeller. Pull it out (it usually lifts out of a round well under the motor). Wipe off slime, remove snails, gravel or plant bits, and check that the blades are not cracked and that the magnet turns the blades with it. Clean the well it sits in with an old toothbrush.
Check the intake tube and any pre-filter sponge for blockages.
Re-prime it:
- Hang-on-back: top the tank up to its normal level, fill the filter box with tank water, then plug it in. Tilt it gently if air gurgles.
- Canister: fill the canister to the brim with tank water, check the hoses are the right way round with no kinks, open all the taps, fill the hoses, then start it. Check the lid seal is seated; a badly seated lid sucks in air.
Do a 25–50% water change with dechlorinated water if the filter was off for many hours or the water smells.
Test ammonia and nitrite now and daily for a week.

What to do next

Feed lightly for a week (every other day, small amounts). Less food means less ammonia while the bacteria recover. See overfeeding.
Keep testing. If ammonia or nitrite show up, do daily partial water changes until they are back to zero. A filter that lost a lot of bacteria can behave like a new tank for a week or two: see cycling.
Replace worn parts. A cracked impeller, a split magnet, a worn shaft or a hardened lid seal should be replaced. These parts are cheap; a new filter costs more.
Retire the filter if the motor housing is cracked, the cord is damaged, or it keeps tripping your safety outlet.

Products that help

Spare impeller for your filter model: the most common part to fail. Keep one in a drawer.
Filter brush set: long thin brushes for intake tubes and hoses, which clog with slime.
Pre-filter sponge on the intake: catches most of the gunk before it reaches the main filter and keeps snails out of the impeller. Rinse it weekly.
Battery-powered air pump: keeps fish (and bagged filter media) oxygenated during a power cut or a filter failure.
Water conditioner: for the emergency water changes.
Ammonia and nitrite test kits: to see whether the bacteria have taken a hit.
Keep on hand: spare sponges (to replace half at a time, never all), spare O-rings or lid seal for a canister, and a bucket used only for the aquarium.

How to prevent it

A simple routine stops almost all of these problems:

Every week: look at the flow. Rinse any intake pre-filter sponge.
Every 2–4 weeks (more often in heavily stocked tanks): rinse the main sponges in a bucket of old tank water until they are "clean enough", not spotless. Brown is fine; clogged is not.
Every 1–3 months: pull and clean the impeller and its well. Limescale and slime on the impeller are a common reason filters fail to restart after a power cut.
Never replace all the media at once. The bacteria live on the sponges and bio-media. If a sponge is falling apart, replace half now and the other half a month later. Filter cartridges sold as "replace monthly" usually don't need replacing; rinse them in tank water until they fall apart.
Keep the water topped up so a hang-on-back filter never sucks air and loses its prime.
After a power cut, check every filter has restarted. If the power was off for more than an hour or two, rinse the media before restarting. See power outage.
Don't overfeed. Uneaten food is what clogs filters fastest.

Why it happened

Most filters stop because of lack of maintenance. Sponges fill with waste until water can't get through, slime builds up on the impeller, or a snail climbs up the intake and jams it. Hang-on-back filters lose their prime when the water level drops or the power goes off; the box drains, and the motor may run dry. Inside a stopped filter, the oxygen runs out, the helpful bacteria die, and the waste starts to rot. Merck's veterinary manual lists a failed biological filter, along with overfeeding, as one of the most common causes of rising ammonia.

When to get help

If ammonia or nitrite stays high after several days of water changes, ask a local fish store or experienced keeper for help. A cup of used sponge from a healthy tank can reseed your filter.
If fish are gasping, lying on the bottom, or have red or inflamed gills, treat it as an emergency: see ammonia and gasping.
If the filter trips your safety outlet or you feel a tingle in the water, stop and read electrical safety.

In the game

Multiple Tank Syndrome doesn't have a clogged filter card yet; the closest is the "Better filtration" card.