Fish jumping out of tank: what to do, why it happens, and the right lid

If you have just found a fish on the floor, put it into a container of tank water right now, even if it looks dead. Fish sometimes recover after being out of the water longer than you'd expect, especially if they landed somewhere damp.

Then do two things: close every gap on top of the tank, and work out why it jumped. A fish that jumps is often escaping something, such as bad water, a bully or a fright.

How to identify it

A fish missing at count time. Check the floor, behind and under the stand, behind the tank, on the lid, inside the filter, and in nearby buckets or nets.
A dried fish found near the tank, often covered in dust.
Splashing or thumps on the lid, especially at night or when lights switch on and off.
Fish dashing at the surface, hanging in top corners, or leaping at feeding time.

Fish most likely to jump: hatchetfish, killifish, bettas, danios, many rasboras, pencilfish, loaches (especially kuhli loaches, which squeeze through tiny gaps), spiny eels, true eels, rope fish, arowanas, and many plecos and catfish at night. Newly added fish of any kind jump more often.

When it's a problem

Normal: some fish leap at food or splash at the surface. That is fine with a lid on.
Warning: fish leaping when nobody is feeding them, or fish crowding at the surface.
Serious: a fish found out of the tank, or several jumping at once. This often points to a water problem.
Emergency: a fish on the floor right now, or fish gasping at the surface and jumping. See gasping.

What to do right now

If the fish is on the floor:

Pick it up gently with wet hands and put it in a container of water from its own tank. Don't drop it back into the main tank yet; weak fish get nipped.
Don't bend or straighten it. A dried fish can be stiff. Forcing it can tear the muscles.
Watch the gills. If the gill covers start moving on their own, its chances are good.
If it isn't breathing, help water over the gills. Hold it gently underwater and move it slowly forwards (never backwards) so water flows into the mouth and out over the gills. Or use a pipette or syringe to push water gently into the mouth. Never put it in front of a strong pump; it can tear the gills.
Keep trying for several minutes. Fish that looked dead have come round after a few minutes.
Once it breathes on its own, put it in a breeder box or a quiet container of tank water with an air stone, out of reach of other fish.

If the fish is completely dry and stiff, with dull, sunken eyes, it has almost certainly died. You can still try for a few minutes, but don't blame yourself if it doesn't recover.

Then, for the tank:

Cover the gaps now, even with plastic wrap or a sheet of glass for tonight.
Count every fish and search for any others.
Test the water: ammonia, nitrite, nitrate, pH and temperature.
Watch the tank for bullying for 10–15 minutes.

What to do next

Watch the survivor for several days. Its skin and slime coat are damaged and its immune system is weak. Keep the water very clean and watch for fungus (white fluffy patches), red sores or fin rot.
Fix the cause:
- Bad water: ammonia, nitrite, low oxygen or a wrong pH make fish try to escape. See ammonia and nitrite.
- Chasing: a bully, or spawning fish chasing each other hard. See aggression.
- Fright: lights snapping on in a dark room, a slammed door, a hand or net coming in suddenly.
- A new tank or new fish: fish are jumpy for the first few weeks. See acclimation.
- Not enough cover: open tanks with no plants make shy fish nervous.

Products that help

A fitted fish tank lid or hood: the single best prevention. Glass or acrylic lids keep in heat and reduce evaporation too.
Mesh cover: a frame with fine netting that lets in light and air but keeps fish in. Good for planted tanks and rimless tanks.
Cut-to-fit lid strips or foam/sponge gap fillers: close the gaps around filters, heaters and cords.
Floating plants: give surface fish cover and make them feel safer.
A light timer, or lights that fade on and off: fewer startled fish.
Breeder box or hospital tank: for a fish that's been out of the water.
Keep on hand: an air stone and pump, water conditioner, and a test kit.

How to prevent it

Use a lid on every tank with jumpers, and on any tank with new fish.
Close the gaps. Most escapes happen through the cut-outs for the filter, heater cable, airline tubing and power cords, and the back corners where lids don't sit flush. Eels and kuhli loaches can get through gaps that look far too small.
Lower the water level 2–5 cm (1–2 inches) below the rim, so jumpers hit the lid instead of going over the edge.
Keep the lid on after maintenance. Many escapes happen the night after a water change, when the lid was left off or put back crooked.
Keep water quality good and the tank calm.
Add plants and hiding places so fish don't panic.
Feed without leaning over the open tank for fish that leap at food, or use a feeding hatch.

Why it happened

Some fish jump naturally: hatchetfish hunt insects at the surface, killifish and bettas come from small pools they sometimes leave in the wild, and many fish leap to escape predators. In aquariums, fish usually jump because something is wrong: poor water, a bully, a sudden fright or a new environment. An open top or a gap round the filter does the rest. Interestingly, field researchers report that hatchetfish rarely jump unless badly frightened, but every hobby guide still recommends a tight lid, and it is the safe choice.

When to get help

If several fish are jumping or gasping, treat it as a water emergency and test now. See gasping.
If the rescued fish develops spreading white, red or fuzzy patches, ask an experienced keeper or a fish vet.
If the fish is badly hurt and clearly suffering, a humane end may be kinder; see dropsy for the clove-oil method.

In the game

Multiple Tank Syndrome doesn't have a jumping fish card yet.