Why Do Fish Jump Out of the Tank? Causes and Prevention

The short answer

Fish jump for several reasons, and care guides link it to sudden scares, chasing tankmates and poor water; some species, such as hatchetfish, killifish and bettas, are known jumpers. A jump becomes an escape only through an open top or a gap, so fit a tight cover and test the water.

There are two halves to the problem: what may have set the fish off, and the opening it got through. You can check both and change what you find. This article covers freshwater tanks.

Why a fish jumps, and when it becomes an escape

Seriously Fish, a fish species database, describes the leap of the hatchetfish as “a modified threat response” that the fish uses in extreme circumstances “much like in many other fishes”. In the wild, the same profile says, hatchetfish jump “to catch flying insects, and to escape potential predators”.

Whether a jump ends on the floor depends on the top of the tank. Aqueon, an aquarium equipment maker, lists it among the reasons fish die: “the aquarium cover gets left open and they jump out”. One commenter in an r/Aquariums thread on the question pointed to the water: “Bad water quality, low oxygen etc. Did you check his water params?”

Triggers and what to check

Trigger	What to check	What helps
Open top or gaps	Lid, and cut-outs for filter, heater cord and airline	A fitted glass top; mesh over the openings
Ammonia or nitrite	A test kit reading (established tank: zero, per Aqueon)	Find and fix the cause (see below)
Water too warm	A thermometer	Heater set for your species; an aerator
A sudden scare	Lights, noise, movement near the tank	Lights off when adding fish; lights changed gradually
Chasing tankmates	Who chases whom	Plants, rocks and decor; separate the fish if it goes on
A jumping species	Its species profile	A tight cover before the fish goes in

Tropical Treasures, a Wyoming aquarium retailer, lists the triggers on its blog: “High ammonia or nitrite can make the water feel toxic, prompting fish to try to escape it”, and “Aggression from tank mates, sudden lights, loud vibrations, or feeling exposed can all trigger a panic jump.” It also names temperature problems and overcrowding.

Check the water first

A jump does not tell you which trigger it was, so test first.

Test ammonia and nitrite

For an established tank, Aqueon’s freshwater water-quality guide says “The only safe ammonia level is zero” and “The only safe nitrite level is zero”. A new tank is different: Aqueon’s nitrogen-cycle guide says “Ammonia levels typically begin to rise by the third day after you’ve added fish to your new aquarium”, and its freshwater water-quality guide advises to stock gradually, feed sparingly and “test water regularly until ammonia and nitrite levels stabilize at zero”. If an established tank reads above zero, our guide to high ammonia in an aquarium covers the usual causes.

Read the thermometer

Aqueon notes that “as water warms up it holds less oxygen”. Ranges differ by species: Aqueon says “most tropical fish prefer 74° to 80° F”, while “Discus, Clown Loaches, Bettas and Rams thrive at temperatures in the mid to upper 80’s”. Check your species’ range and set the heater “for the middle of your fishes’ preferred temperature range”.

Cool a hot tank with the cover on

Aqueon’s hot-weather steps include an aerator “to improve gas exchange at the surface”, turning off the aquarium light and closing the curtains on a sunny window, and it says to “make any fish temperature changes in your aquarium gradual”. Its fan tip means opening the cover, with the water lowered “an inch or two to prevent fish from jumping out”; with known jumpers or a fish that has jumped, keep the cover on and use the other steps. Our guide to why aquarium water needs oxygen explains surface agitation and air stones.

Warning

If fish look sick, are injured or are dying, test the water now and contact an aquatic veterinarian. A cover helps keep fish in the tank; it does not fix a water or health problem.

Species known to jump

Some fish need a cover even when the water is perfect.

Hatchetfish. Aquarium Co-Op, an aquarium retailer, says the tank “must have a tight-fitting lid or hood because they will jump out of the narrowest slot they can find”. Seriously Fish is more measured: anecdotal evidence suggests “gasteropelecids rarely jump and they also tend not to do so in aquaria, even when being chased with a net”. It still warns that “leaving the tank cover open during darkness is likely to result in one or more hatchetfish on the floor by morning”.
Killifish. Seriously Fish’s profile of the striped panchax (Aplocheilus lineatus) says “The tank should have a tightly fitting cover as it is an excellent jumper”; its profiles of Gardner’s killifish (“a prodigious jumper”) and the lyretail killifish (“the cover must be very tight fitting”) say the same.
Bettas. Seriously Fish says to keep the tank “well-covered and do not fill it to the top”, because bettas need “occasional access to the layer of humid air that will form above the water surface”, and the species “is an excellent jumper”.

How to keep fish in the tank

Fit a cover made for the tank

Aqueon’s Glass Tops, for example, have “a silicone hinge, handle and removable back strip that can be easily modified to accommodate filters and other equipment”. Our guide to why an aquarium should be covered compares glass, plastic and mesh lids.

Close every gap

Cut the back strip around the filter, heater cord and airline. Aquarium Co-Op’s advice: “Make sure the openings are very tight so that fish and invertebrates cannot escape.” For the openings that are left, Co-Op suggests covering them “with craft mesh or another material”, and Tropical Treasures adds that with a mesh cover you should “make sure its openings are smaller than the fish”.

Do not rely on a low water line

Tropical Treasures cautions: “A lower waterline may provide extra margin but is not a substitute for a fitted lid.” For bettas, keep the cover on and leave the air gap under it.

Give fish somewhere to go

Aqueon says most fish “need cover in the form of plants, rocks and other decorations to feel safe” and may “hide or get picked on if there isn’t enough structure in the tank”. If one fish keeps chasing another, Tropical Treasures advises: “separate incompatible fish if the behavior continues”.

Keep the lights and arrivals calm

A common method for new fish, per Aqueon, is “to turn off the aquarium light and float the bag in your tank”, adding tank water to the bag “every few minutes for half an hour”. Aqueon adds: “Never allow store water to enter your aquarium, discard it after you release the fish.” Tropical Treasures suggests you “turn tank lights on and off gradually where possible”.

If you find a fish on the floor

Practical Fishkeeping, a fishkeeping magazine, says “how you handle the fish immediately on discovery is crucial to its survival”.

Do not bend it

The magazine warns: “Do not, under any circumstances, try to bend or otherwise ‘loosen it up’.”

Get it into tank water, safe from the other fish

Practical Fishkeeping’s first step is to “get the fish back into some water, but in a safe environment”: “Simply dropping the fish back in to the tank is a bad idea. It will be vulnerable to the attentions of curious tank mates who will nip and nibble it.” So hold it in your hands under the tank water: “Submerge the fish into the tank in your hands and look for a breathing response.” Keep it away from strong currents: “Do not hold the fish in front of a powerful flow of water like a powerhead, as this will tear the gills.”

If it breathes on its own, move it to an aerated spot

The magazine suggests “a body of tank water in which you have an air pump”; “A breeding trap will do the job inside the aquarium, but heavy aeration inside the trap will be vital”.

If the gills are barely moving or still, call an aquatic veterinarian

Practical Fishkeeping says that then “the fish needs more help”. This site does not cover revival steps: contact an aquatic veterinarian, and until then keep the fish in tank water, out of reach of tankmates.

Watch it, then fix the cause

A fish that recovers needs watching “for a few more days”; if it looks unwell or injured, contact an aquatic veterinarian. Test the water and find the gap it got through.

Ready to buy?

Our roundup of freshwater aquarium test kits lists kits for the ammonia and nitrite check, and our Water Care & Testing section gathers the test kits and conditioners.

Frequently Asked Questions

Q. Why do fish jump out of the tank at night?

Tropical Treasures lists “Sudden lights, vibrations, nocturnal activity, bullying, and a species’ natural jumping behavior” as possible factors. Seriously Fish’s hatchetfish warning is about a cover left open in the dark: keep it on overnight.

Q. Is it normal for fish to jump?

Some species are able to jump: Seriously Fish’s profiles describe hatchetfish, several killifish and bettas that way. For any fish that jumps, check the water, the tankmates and the setup before you put it down to the species.

Q. Why did my fish jump after a water change or tank cleaning?

Check what changed. Aqueon’s routine is tap water treated with a conditioner “before adding it to the aquarium”, replacement water “the same temperature as the aquarium water”, and: “Avoid water changes of more than 25% as larger water changes can stress your fish and upset the balance in your aquarium.” If the cover came off for cleaning, put it back.

Q. Can a fish survive after jumping out of the tank?

Sometimes. Practical Fishkeeping says “Caught early enough a fish can recover, even from a surprisingly dry state.” Follow the steps above, and contact an aquatic veterinarian if it is not breathing well, is hurt or looks unwell.

Sources (17)

Aquarium Tales Team

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