Fish Tank Filter Not Working? Clean, Prime, or Replace the Media
A dead or weak aquarium filter is usually clogged media, a dirty impeller or an air lock. Check them in order, then replace only what is worn.
By Jamilur Rahman Updated November 5, 2026 · 8 min read
- Time
- 20 min
- Cost
- Under $20
- Difficulty
- Easy
In this guide
Quick answer
If your fish tank filter is not working, the cause is usually a clogged cartridge, a dirty or stuck impeller, or a pump that lost its prime. Unplug it, check the water level, rinse the media in old tank water, clean the impeller and refill the filter before you plug it back in. If the flow stays weak, a fresh cartridge or a new impeller is the fix, and both are cheap.
Free fix vs. permanent fix
Clean, prime and restart
- Unplug the filter and check the water level
- Rinse the cartridge or sponge in a bucket of old tank water
- Clean the impeller and its well with a soft brush
- Fill the filter with tank water to prime it, then plug it in
How long it lasts: Often a few weeks, until the media clogs again. A worn impeller or a cartridge that is falling apart will not hold.
Fresh filter cartridges that match your model
Flow fades slowly as a cartridge fills with waste, and rinsing never makes it like new. A fresh cartridge made for your exact model gives the water a clear path again and takes strain off the impeller, which is the part that stalls when media clogs.
What to get: Replacement filter cartridges for your filter model
How long it lasts: Each cartridge works until it clogs, so replace it on the schedule in your manual.
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What you'll need
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Bucket of old tank water
Rinse media in this, not tap water, to keep the helpful bacteria alive
Free -
Soft brush or old toothbrush
For the impeller, its well and the intake tube
You have it - Our pick Illustration
Replacement filter cartridges
Must match your filter model exactly
- Illustration
Cut-to-fit filter sponge or floss
Rinse and reuse, so it costs less over time
- Illustration
Replacement impeller and shaft
Only if the old one is cracked, chipped or will not spin
Why it happens
A clogged cartridge or sponge
Fish waste, leftover food and plant bits fill the media. Water backs up and the flow drops, then the filter may stop pulling water through at all.
Water level too low
A hang-on-back filter needs its intake tube fully under water. After evaporation the intake sucks air, the pump runs dry and it rattles or stops.
An air lock or lost prime
After a power cut, a water change or a cleaning, air can sit in the pump chamber. The motor hums, but no water moves until the chamber refills.
A stuck, dirty or worn impeller
The impeller is the small spinning rod with a magnet. Snail eggs, gravel, plant debris and mineral scale can jam it, and a cracked or chipped one spins badly or not at all.
A power problem
A tripped GFCI outlet, a loose plug or a power strip that is switched off means no flow. Check these before you take anything apart.
Try this first (free)
Do these in order, starting with the simplest.
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1. Unplug it and check the water
Always unplug before touching a filter. Top the tank off so the water sits above the intake and, on a hang-on-back model, above the spill lip.
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2. Check the outlet and cord
Test the outlet with a lamp and look for a tripped GFCI. Inspect the cord for cuts. If the cord is damaged, replace the filter and do not use it.
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3. Rinse the media in old tank water
Tool used: Bucket of old tank waterTake out the cartridge or sponge and swish it in a bucket of water you just drained from the tank. Squeeze until the water runs mostly clear. Tap water can kill the helpful bacteria that live in the media.
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4. Pull and clean the impeller
Tool used: Soft brush or old toothbrushSlide out the impeller, wipe the rod and brush the well it sits in to remove slime and snail eggs. It should spin freely by hand. If it is cracked or the magnet is chipped, replace it.
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5. Prime the filter and restart
Fill the housing with tank water, set the media back and plug it in. A hang-on-back should pull water within seconds. If it hums but does not pull, tap the body to let trapped air out.
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6. Swap the media if it is torn or slimy
Tool used: Replacement filter cartridgesIf the cartridge is falling apart or clogs again within days, replace it. Place the old one beside the new one for a week or two so the helpful bacteria move across.
IllustrationCheck price on AmazonTool for this step
Replacement filter cartridges · Must match your filter model exactly
When free fixes aren't enough
Rinsing helps a clogged cartridge for a while, but the media wears out. If it clogs again within days, falls apart in your hand or stays grey after rinsing, replace it. If the media is fine and the water level is right but the motor hums without pulling water, or the impeller is chipped, replace the impeller. A filter that stays off for hours stops feeding the good bacteria, so test for ammonia and do a partial water change if the fish gasp at the surface. If the motor is dead or the cord is damaged, replace the whole filter. Never open a filter that is plugged in.
Buy the fix
Still stuck after the free fixes? These are the products we would look at.
| Pick | Product | Fixes | Who it's for | Buy |
|---|---|---|---|---|
| Best pick | Replacement filter cartridges for your filter model | weak flow from a clogged or worn cartridge | Anyone whose cartridge is slimy, torn, past the time in the manual or still slow after rinsing. | Check price |
| Budget pick | Cut-to-fit aquarium filter sponge or floss | clogged media, at a lower cost over time | Owners who would rather rinse and reuse than buy cartridges again and again, and do not mind trimming to fit. | Check price |
Replacement filter cartridges for your filter model
Fixes: weak flow from a clogged or worn cartridge
Look for: Listed as compatible with your exact filter model or size; A mechanical layer that catches debris, plus carbon or a biological layer; A multipack so you can swap on schedule.
- Who it's for
- Anyone whose cartridge is slimy, torn, past the time in the manual or still slow after rinsing.
- Honest downside
- Your filter takes a sponge or loose media instead of cartridges. Check the manual before you buy.
Cut-to-fit aquarium filter sponge or floss
Fixes: clogged media, at a lower cost over time
Look for: Medium or coarse pores that water passes through easily; A thick sheet you can trim to your filter's shape; Labeled aquarium-safe, with no dye or coating.
- Who it's for
- Owners who would rather rinse and reuse than buy cartridges again and again, and do not mind trimming to fit.
- Honest downside
- Your filter only works with a fixed-shape cartridge. Water must not be able to bypass the media.
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How we picked: How we picked: this page is based on aquarium filter manuals, water-care guides from fish-keeping groups, and the problems and fixes people report in filter reviews. We do not test products by hand.
How to fix a fish tank filter, by type
- Hang-on-back: check that the intake is under water, then rinse the cartridge, clean the impeller well and prime it.
- Internal or submersible: unplug it, lift it out, clean the intake grille and impeller, and make sure it is fully under water.
- Canister: turn off the valves and unplug it before you open it. After cleaning, refill and bleed the air out so it can prime.
- Sponge filter: these run off an air pump, so check the airline and pump rather than the sponge. See the related page on tanks that stop bubbling.
Find your model number before you buy
The model number is usually on a sticker on the back or base of the filter, or in the manual. Cartridges and impellers are not universal, so match the model, or the size letter if the listing uses one. If you cannot find it, measure the old cartridge and compare it with the listing photos.
What to skip
Scrubbing media under the tap, or replacing everything at once
Hosing a cartridge under the tap or scrubbing it spotless wipes out the bacteria that handle fish waste. Replacing the cartridge, sponge and gravel in one go can cause an ammonia spike. Rinse in tank water and change one thing at a time.
Frequently asked questions
How do I fix a fish tank filter that is not working?
Why is my fish tank filter running but no water is coming out?
Can I rinse filter media under the tap?
How long can fish go without a working filter?
How often should I replace filter cartridges?
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