Equipment
The filter output has slowed to a trickle
Flow falls for mechanical reasons almost every time: a blocked intake, media saturated with debris, or an impeller wrapped in something. Unplug the filter before you open it, work through the path the water takes from intake to outflow, and rinse anything you remove in a bucket of tank water rather than under the tap.
Act this week
Test these first
Before anything goes into the tank and before anything comes out of it, get these numbers. They cost fifteen minutes and they decide everything that follows.
- Ammonia — total ammonia — the ammonium ion plus free ammonia, added together
- Nitrite — nitrite ion, the intermediate product between ammonia and nitrate
What is usually behind it
| Cause | How to confirm | Time to resolve |
|---|---|---|
| Intake strainer blocked | Look at it. Plant matter, a leaf, snail shells or a build-up of fine debris across the slots is the most common single cause and the easiest to miss because the strainer is usually behind something. | Five minutes, and flow returns immediately |
| Mechanical media saturated with debris | Flow has declined gradually over two or three weeks, and the floss or sponge is visibly loaded when you take it out. | Ten minutes, with full flow restored the same day |
| Impeller fouled or worn | Flow is weak even with clean media and a clear intake, and the filter may be noisier than usual. Unplug it, remove the impeller, and look for hair, plant fiber or a chipped magnet shaft. | Twenty minutes to clean, or a few days if a part has to be ordered |
| Air trapped in the pump chamber | The problem started immediately after a water change or after the filter was restarted, and the unit is making a rattling or sucking sound rather than a hum. | Minutes |
| Too much media crammed into the housing | The filter has been packed with extra sponge, carbon bags and resin until the chamber is full. Water takes the easiest route, and if there is not one it slows down. | Immediate |
Intake strainer blocked
How to confirmLook at it. Plant matter, a leaf, snail shells or a build-up of fine debris across the slots is the most common single cause and the easiest to miss because the strainer is usually behind something.
What to doUnplug the filter, lift the strainer, and clear it by hand or with a soft brush. Make this a standing item on your water change routine.
Time to resolve: Five minutes, and flow returns immediately
Mechanical media saturated with debris
How to confirmFlow has declined gradually over two or three weeks, and the floss or sponge is visibly loaded when you take it out.
What to doRinse it in a bucket of water you just siphoned from the tank, squeezing until the worst is out, and put it straight back. Never rinse it under the tap — the residual disinfectant in municipal water kills the bacteria living on it.
Time to resolve: Ten minutes, with full flow restored the same day
Impeller fouled or worn
How to confirmFlow is weak even with clean media and a clear intake, and the filter may be noisier than usual. Unplug it, remove the impeller, and look for hair, plant fiber or a chipped magnet shaft.
What to doClean the impeller and its housing with a soft brush. If the shaft is chipped or the blades are worn, replace the impeller assembly — it is a wear part on almost every model.
Time to resolve: Twenty minutes to clean, or a few days if a part has to be ordered
Air trapped in the pump chamber
How to confirmThe problem started immediately after a water change or after the filter was restarted, and the unit is making a rattling or sucking sound rather than a hum.
What to doUnplug it, tilt the housing gently to release the trapped air, and restart. Check that the water level is high enough to keep the intake fully submerged, since a low level pulls air in continuously.
Time to resolve: Minutes
Too much media crammed into the housing
How to confirmThe filter has been packed with extra sponge, carbon bags and resin until the chamber is full. Water takes the easiest route, and if there is not one it slows down.
What to doTake some out. A filter with less media and good flow biologically outperforms a filter with more media and no flow, because the bacteria need water moving past them.
Time to resolve: Immediate
What not to do
These are the moves that make this particular problem worse, and they are all common. Most of them come from wanting to do something substantial when the right answer is something small.
- Do not open a filter that is still plugged in. Unplug it at the outlet first, every time, without exception.
- Do not rinse biological media under a running tap, and do not replace all of it in one session. Either one can flatten your cycle within days.
- Do not run the filter dry while you work out what is wrong. Unplug it rather than leaving it running with no water.
- Do not solve slow flow by buying a bigger filter and leaving the blocked one in place.
When this stops being a water question
Reduced flow means reduced surface agitation, so fish gathering at the surface during this is an oxygen sign rather than a curiosity. Restore agitation first. If a fish stays unwell once flow and readings are normal, that is a veterinary matter and an aquatic veterinarian is the right call.
Keeping it from coming back
Almost all of this is prevented by a two-minute habit: at every water change, unplug the filter, clear the intake strainer, and give the mechanical media a squeeze in the bucket of water you just removed. Stagger any media replacement so an established piece always stays in the housing, keep the water level high enough that the intake never sips air, and keep a spare impeller for the model you own if it is the kind that wears.
Reference for this page
Parameters
Equipment involved
- Filtersturning water over and housing the bacteria
- Filter Media & Cartridgeswhat actually does the filtering, and when it gets replaced
- Test Kitsknowing the number instead of guessing it
Related problems
Watch it
Act this week
Act this week
Published sources
- Electrical Safety Foundation International
- US EPA — Drinking Water Requirements for States and Public Water Systems
- USGS Water Science School — Dissolved Oxygen and Water
Published figures on this page come from the documents above. Where a value depends on species, pH or temperature, that is stated rather than averaged into a single number.