Filter intake sponge: stop your filter sucking up fish, fry and shrimp

If a fish is stuck to your filter intake, unplug the filter first, then free the fish gently. Then fit a pre-filter sponge over the intake, which stops nearly all of these accidents for very little money.

Keep one more thing in mind: a healthy adult fish rarely gets pinned. A fish stuck to the intake is often already weak, sick or exhausted, so after you rescue it, look for the real problem.

How to identify it

A fish flattened against the intake strainer (the slotted tube that sucks water into the filter), unable to swim away.
A betta's long fins pulled into the slots, or torn or shredded fins near the intake.
Fry or shrimp numbers dropping for no clear reason. Open the filter: you may find them, alive or dead, in the box or canister.
Small fish, such as kuhli loaches, wedged inside the intake tube.
Fish that always hang near the intake, seemingly too tired to fight the pull.

When it's a problem

Normal: fish swim past the intake easily, and some even like resting in the current nearby.
Early warning: a fish brushes the intake and struggles a moment, fins get tugged, or you have fry or shrimp and no guard on the intake.
Serious: a fish found pinned, missing fry or shrimp, torn fins.
Emergency: a fish pinned and not moving, or a fish inside the filter.

What to do right now

Unplug the filter. Don't try to pull a fish off a running intake; you will tear its skin, fins or gills.
Wait a few seconds for the suction to stop. Many fish float free by themselves.
If it is still stuck, ease it off gently with a wet hand or a soft net. For fish inside the tube or filter box, take the intake apart and tip the fish into a container of tank water.
Check whether it is breathing. Watch the gill covers. A fish that has been pinned for a while may lie still for a few minutes before it recovers.
Put it somewhere calm with clean water. A breeder box or net in the main tank works, or a hospital tank filled with water from the main tank. Add an air stone, keep the lights low, and keep other fish away from it.
Check its injuries: scraped skin, missing scales, split fins, red or white patches, or damaged eyes.
Slow the flow or guard the intake before you plug the filter back in, even if it's only a sponge or a fine mesh bag held on with a rubber band for now.
Test the water (ammonia, nitrite, nitrate, temperature) to look for the reason the fish was weak.

What to do next

Keep the injured fish clean and calm for a few days. Clean water is the best treatment for scrapes and torn fins. Watch for fin rot, fungus (white cottony tufts) or columnaris appearing on the damaged areas.
Look for the real cause. Ask yourself:
- Is the water quality poor? Test for ammonia and nitrite.
- Is it too cold or too hot? See cold and heater overheating.
- Is the fish being bullied into a corner? See aggression.
- Is the fish thin, bloated, or showing signs of illness? See fish dying and swim bladder.
- Is it a long-finned betta tired out by strong current? See betta fin rot.
Match the filter to your fish. If the filter is pushing small or weak fish around the tank, it is too strong for them, or the outlet needs a baffle.

Products that help

Pre-filter (intake) sponge: a foam sleeve that slips over the intake tube. It keeps fish, fry and shrimp out, spreads the suction over a larger area, and grows helpful bacteria. Use one on any hang-on-back or canister filter in a tank with small fish, fry, shrimp, bettas or loaches.
Fine stainless-steel or plastic mesh intake guard: an alternative to a sponge that clogs more slowly. For baby shrimp, choose a very fine mesh.
Sponge filter (air-driven): a gentle, safe filter for fry tanks, shrimp tanks and betta tanks, with no impeller at all.
Filter with adjustable flow: lets you turn the flow down for slow swimmers.
Outlet baffle (a piece of sponge or a spray bar): spreads the outflow so the whole tank isn't a fast river.
Breeder box or hospital tank: for a rescued fish to recover in.
Keep on hand: a spare intake sponge, a soft net, water conditioner and a test kit.

How to prevent it

Fit an intake sponge from day one in any tank with fry, shrimp, small fish (such as tetras, rasboras or kuhli loaches), bettas or plecos small enough to go up the tube.
Rinse the sponge weekly in a bucket of tank water. A clogged sponge slows the flow through the whole filter, and some keepers find thin sponges clog within days; a larger sponge clogs more slowly.
Know its limits. Depending on suction and the size of the animal, tiny fry, baby shrimp and small snails can still sometimes get through foam. For newborn fry, an air-driven sponge filter is safest.
Size the flow to the fish. Bettas come from calm, slow water; strong filters tire them and tear their fins. Long-finned fancy guppies and goldfish are also poor swimmers. Fast-water fish (many plecos, hillstream loaches, danios) are happy with more flow.
Check the tank every day and count your fish. A weak fish near the intake is an early warning.
Keep the water clean. Healthy fish rarely get stuck.

Why it happened

A filter intake sucks water hard through a small area. Small, young, long-finned or weak fish can't always swim against it. Fry and baby shrimp are small enough to be sucked straight through the slots. Some fish, such as kuhli loaches, push into tubes on their own. Most often, though, a fish ends up pinned because it was already too weak to swim, from bad water, cold, disease, bullying or old age, and the filter was just the place where it gave out.

When to get help

If the rescued fish isn't swimming normally within a day, stops eating, or its wounds turn white, fuzzy or red and spreading, ask an experienced keeper or a fish vet.
If other fish are also weak, gasping or hanging near the intake, it is probably a water problem; see gasping and ammonia.
If the fish is very badly injured and not recovering, a humane end may be kinder; see dropsy for the clove-oil method.

In the game

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