Fish out of the water: rescuing a jumped fish and the days after

Quick answer

Finding a fish on the floor is the aquarium version of a road accident: a sudden physical catastrophe where what you do in the first minute decides most of the outcome. Fish do not get hit by cars, but they do jump, they get spilled during water changes, and they get stranded in overflow boxes and behind decor.

Two things are happening to a fish out of water, and only one of them is obvious. It is suffocating, because gill filaments need water to hold them apart and they collapse into a useless clump in air. And it is losing its slime coat wherever it touches a dry surface, which is why a fish that seems to recover fully can die of infection three days later.

Wet everything that will touch the fish before you touch it, including your hands.

• Wet your hands before you pick the fish up. Dry hands strip the protective mucus layer in the places you grip.
• Return it to its own tank water, at its own temperature. Never into cold tap water, and never into untreated tap water.
• If it cannot stay upright, support it gently in the water facing into a gentle current so water flows front to back over the gills.
• A fish that looks fine after ten minutes is not out of danger. The infections that follow this appear over the next few days.
• Never move a fish backwards through water to "push water over the gills". Water must enter at the mouth and leave at the gills.

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Do not put the fish into fresh tap water.

Tap water in most supply systems contains chlorine or chloramine, and both destroy gill tissue on contact. A fish whose gills have already been damaged by air exposure has no reserve at all. Use water from its own tank, or new water that has been dechlorinated and brought to the same temperature. If nothing else is available, use the tank water even if you think the tank water caused the jump.
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Decide in 60 seconds

Why air is lethal to a fish

The gill of a fish is a stack of very thin, very close filaments, each carrying rows of even finer plates. Water flowing between them presents an enormous surface area to a thin blood supply. In water, buoyancy keeps those structures separated.

In air, surface tension pulls them together. The stack collapses into a solid mass, the surface area available for gas exchange drops to a fraction of what it was, and the fish cannot extract oxygen from air even though it is surrounded by it. Adding to that, the exposed gill surface dries and the delicate tissue is damaged directly.

This is why a fish out of water opens and closes its mouth desperately and gets nowhere. It is not choking. Its gas exchange surface has physically folded up.

The second injury is to the skin. A fish is coated in a mucus layer that is its immune barrier, its physical barrier and part of its water balance system. Contact with a carpet, a wooden floor, a dry towel or a dry hand strips it off in patches. Underneath is unprotected skin, and the water a fish lives in is full of bacteria and fungal spores that are harmless to intact skin and opportunistic on damaged skin.

That is the mechanism behind the most frustrating outcome in this scenario: a fish that is rescued, swims normally within the hour, and then develops a fungal patch or an ulcer three to five days later.

Species differences that change how long you have

Not all fish are equally helpless in air.

Labyrinth fish, which include bettas, gouramis and paradise fish, have an accessory breathing organ that lets them take oxygen from atmospheric air. They tolerate time out of water far better than most aquarium species, and a betta found on the floor is often recoverable when a tetra in the same situation would not be.

Several catfish, including Corydoras, gulp air at the surface and absorb oxygen through the gut. Many loaches, eels and killifish also handle brief air exposure well, and some travel over damp ground in the wild.

Goldfish and koi are relatively robust, partly because they are large and their gill surfaces do not dry as fast.

Small, thin-bodied, fast-metabolism species such as tetras, rasboras and danios are the least tolerant. So are marine fish, which are also fighting osmotic stress the whole time.

Body size matters on its own. A large fish has more water in its gill chamber and a lower surface-to-volume ratio, so it dries more slowly.

None of this means waiting is acceptable for any species. It means that finding a betta on the carpet is worth a full rescue attempt even when it looks hopeless.

A jug, a clean container and a dechlorinator kept together mean you can set up recovery water in under a minute rather than searching for things.

The rescue, step by step

Wet your hands. This takes two seconds and prevents a large part of the slime coat damage. If you have a soft net or a clean container, wet that instead.

Lift with support under the body. Never by the tail, never by a fin, never with a pinch grip. Two cupped wet hands, or slide a wet net or a piece of wet card underneath.

Do not rinse it under the tap. That is chlorinated water on damaged gills.

Put it back into its own tank water at its own temperature. If the tank has been drained, or the water is suspect, use dechlorinated water matched to the same temperature. A temperature shock on top of everything else is a separate injury.

Lower it in gently rather than dropping it.

If it cannot stay upright, hold it. Support it lightly in an upright position near an airstone or a filter outflow so that water moves past the head from front to back. This is the direction water normally travels: in at the mouth, out over the gills. Moving a fish backwards through the water forces water the wrong way through the gill chamber and does harm.

Increase aeration. An airstone, a raised filter outflow, or a spray bar aimed at the surface. Oxygen is the one thing you can add immediately, and a fish with damaged gills needs the water to be as saturated as possible.

Turn the lights off and reduce disturbance. Do not net it, chase it, or keep lifting it to check.

Do not force water into the mouth with a syringe unless a vet who sees fish has told you to. It is easy to do more damage.

Give it time. Recovery from lying on the bottom to swimming normally can take a long stretch. Judge by the gills continuing to move and by gradual improvement, not by a clock.

The days after are the dangerous part

Once the fish is swimming, the job changes from rescue to preventing infection in stripped skin.

Keep the water immaculate. Test ammonia and nitrite, and do small partial water changes with dechlorinated, temperature-matched water rather than one large one. Any ammonia at all is an added burn on damaged gills.

Keep the temperature stable and appropriate for the species. Do not raise it "to help", because warmer water holds less oxygen and the fish is already short of gas exchange capacity.

Watch the areas that touched the floor. The typical progression is a dull or grey patch, then a white cottony growth, or a reddened area that becomes an open ulcer. Cloudy eye, ragged fin edges and a hazy sheen over the body are all worth acting on.

Feed lightly and only what is eaten. A fish that is not eating will not be helped by food decaying in the tank.

Be careful about what you add to the water. Salt is often suggested and is genuinely useful for some fish, but it is not safe for many scaleless species, several loaches and catfish, or planted and invertebrate tanks. Medication chosen for the wrong species or dosed on the wrong water volume kills more fish than the original injury. Ask before adding anything.

Watch for delayed physical damage too. A fish that hit furniture on the way down can have a spinal injury that shows up as a bend in the body, an abnormal swimming posture, or an inability to hold position, and jaw damage that shows up as an inability to feed.

Wet hands, both of them, supporting the whole body, and the shortest possible time in air. Never a pinch grip and never by the tail.

Why it happened, and the part that is not about the lid

A gap in the lid explains how a fish left the tank. It never explains why it tried.

Water quality. Ammonia and nitrite both damage gills and make a fish frantic. A fish trying to escape its own water is a genuine and common reason for jumping, and it is the first thing to test.

Low oxygen. Warm water, an overstocked tank, a stalled filter or a surface with no movement. Look for other fish hanging near the surface.

Temperature. Too warm, too cold, or a heater that has failed on.

Aggression. A fish being chased will leave the water to escape. Watch the tank quietly for a while rather than assuming peace.

Current. A filter return aimed across the surface can launch a small fish, particularly at a corner.

Species and behaviour. Hatchetfish, killifish, gobies, arowana, bettas and many others jump as normal behaviour. Some species jump when spawning. Startle from a light being switched on in a dark room is a common trigger.

If the water tests clean, the temperature is right and the tank is calm, then the lid is the whole story. If any of those is off, fixing the lid alone leaves the fish trying.

What never to do

Do not handle a fish with dry hands, a dry towel, or a dry net.

Do not hold it by the tail or by a fin.

Do not rinse it under a tap or put it into untreated tap water.

Do not move it backwards through the water.

Do not put a tropical fish into cold water or a coldwater fish into warm water to "revive" it.

Do not squeeze the body, massage it, or blow into its mouth.

Do not add medication, salt, or a stress-coat product without checking that it is safe for that species and that tank, including any invertebrates and plants.

Do not raise the temperature to speed recovery.

Do not keep lifting it out to check on it.

Do not decide it is dead because it is not moving. Decide it is dead if the body is stiff, the eyes are sunken or opaque, and the surface is dried.

When to get help

Contact a vet or an aquatic specialist who sees fish if the fish cannot stay upright after several hours, is breathing very rapidly with flared gill covers, has an obvious bend in the body or an injured jaw, has an open wound or a growing white or red patch, or has not eaten for several days.

The target is a fish back in stable, well-oxygenated water of its own, with the lights low and nothing else changing.

If the fish is severely dried, unable to right itself after a long period, or clearly in distress with no improvement, discuss humane euthanasia rather than prolonging it. There are correct methods for fish and improvised ones cause suffering, so ask rather than guess.

Have ready the species, the tank volume, how long the fish was out and on what surface, the current ammonia, nitrite, nitrate, pH and temperature readings, when the tank was last changed or disturbed, what else lives in the tank, and clear photographs of any damaged area taken through the glass rather than by lifting the fish out again.