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Worst case: Act today4 possible causes

The filter flow has dropped

Output from the filter is noticeably weaker than it was, or has stopped.

Short answer

Reduced flow is nearly always mechanical — clogged media, a blocked intake, an airlock or a worn impeller. It matters biologically too, because a filter starved of flow loses oxygen and biological capacity.

Check these first, in this order

  1. Is the intake blocked by plants, snails or debris?
  2. When was the mechanical media last rinsed?
  3. Is there air trapped in the filter body?
  4. Does the motor sound normal?

Possible causes

Ordered by urgency, then by how commonly each one turns out to be the answer. The discriminator is the observation that separates this cause from the others — use it before acting.

Blocked intake

Act todayvery common

How to tell: Sudden drop. Plant matter, a snail or a fish is on the strainer.

Check

  • Inspect the intake

Response

  • Clear it
  • Fit a pre-filter sponge, which also protects small fish and shrimp

Airlock in a canister filter

Act todaycommon

How to tell: Follows maintenance. The filter hums but moves little water, and there is a gurgling sound.

Check

  • Listen for gurgling
  • Whether it started after a service

Response

  • Tilt or rock the canister to release trapped air
  • Check hose connections for leaks drawing air in

Clogged mechanical media

Act within daysvery common

How to tell: Flow has declined gradually over weeks. Fine floss or sponge is visibly loaded.

Check

  • Inspect the media
  • How long since it was last rinsed

Response

  • Rinse coarse media in tank water
  • Re-order the media coarse-to-fine so fine material is last

Worn or fouled impeller

Act within dayscommon

How to tell: Rattling or grinding noise, or flow that does not recover after cleaning the media.

Check

  • Remove and inspect the impeller and its shaft
  • Look for calcium deposits or a chipped magnet

Response

  • Clean the impeller and its housing
  • Replace a worn impeller or a broken shaft

What not to do

  • Rinsing biological media under the tap while you are at it
  • Replacing all media because the flow dropped

Prevention

  • Rinse mechanical media on a schedule
  • Fit a pre-filter sponge on the intake
  • Clean the impeller every few months