piranha behavior in a tank

The Top 13 Signs of Normal/Abnormal Behavior For Piranha

When owning Piranha, like any fish, it’s a good idea to learn their unique peculiarities. They are much maligned and misunderstood fish. Learning to understand them brings great benefits.

So what is normal behavior for Piranha in a tank? In short, if Piranha are given a good amount of space, have good quality water, a varied diet, low lighting, and a noise free environment you should see healthy fish. While not a guarantee it is the best you can do. Signs of stressed fish, lead to combat and eventually a fatality in the tank. Early signs are often repeated chomping and fin nipping. If you see bits of fin missing, it’s always best to rethink what could be wrong.

piranha behavior in a tank


Piranha is a fish predominantly known for its ferocious behavior. Movies and media have done much to cement the idea that they are bloodthirsty killers. In fact, as an omnivore, they can be rather shy and timid, especially in an aquarium.

They are considered a small to medium sized fish, bred in captivity, but native to the waterways of Latin America. They are usually found in the lowlands of the Amazon basin.

The most distinctive feature is quite naturally, those interlocking razor sharp teeth that replace in quarters.

Although a lot of the Amazon has yet to be explored, there are believed to be between 30 and 60 species of the genus, but it depends upon how they are classified, as some are not ‘true Piranha’.

The Amazon river is said to contain a third of the available species. The black Piranha is sad to grow to a massive 24 inches, but most Piranha are much smaller. The average size is around 8 inches, but the famous Red-Bellied Piranha is around 12 to 14 inches in the wild and 10 to 12 inches in an aquarium. Most Piranha are slightly smaller when they are in captivity.

Largely thanks to either Hollywood or the media exaggerating attacks, people have an unreasonable fear of Piranhas. In the wild they will rarely attack a large live animal in the water, fearing it to be a predator. Fisherman wade into the Amazon river all the time to place their nets.

They are not studied a great deal in the wild, with very few studies have been performed, but there are now quite a few anecdotal pieces of evidence about how they behave.

piranha behavior in a tank close up

Normal Behavior For Piranha

When trying to figure what may be unusual behaviour in a Piranha aquarium, it’s important to note that they can be quite complex animals. There are many factors that can increase their stress levels, from brightly lit aquariums, too noisy, confined spacing to an unvaried diet or water cleanliness.

Stressed fish can seek shelter, or be combative towards other fish, so getting the basics of keeping a healthy fish right goes a long way to understanding abnormal behaviour.

Nonetheless, it’s important to be observant to their routines so you can understand their moods and keep abreast of their health.

So here’s a list of normal Piranha behaviour, and what might be happening.

Tilting the head: If Piranha aim slightly towards the gravel, or bottom of the tank while lifting the tail, this is a sign of aggression. It usually happens if there is a particularly aggressive and dominant fish, but can be because the tank is too small. They are warning other fish not to invade their space. Check you have at least 20 gallons per fish if you have a schooling species like the Red-Bellied Piranha.

Pale appearance: Normally, a Piranha might have a very colorful chest, like the famous Red-Bellied Piranha. It could be that they are too young, or they are stressed. Piranha slightly lose their color overnight as well.

Turning on their sides: If they do this it will normally be because they wish to rub themselves on the gravel bottom. This could be due to unclean water, an infection or a parasite that needs to be removed.

Mouth open: Normally you don’t see Piranha teeth. If their mouths are open and there’s quick movement from side to side, this is like a bull getting ready to charge. They are preparing for conflict. Again, consider whether there is enough room, decoration or cover for them to relieve their stress.

Chomping: The act of biting repeatedly, is either a warning to other fish, the final sign before an attack, or hunger. If they are normally stress free then you should feed them. If they are fed regularly, they could be stressed if you see it often. Again check tank size rule, decoration, water quality or available cover.

Tank circling: Piranha can do this if one wants to dominate the tank. They may be declaring territory. Either that or something is wrong, normally a lack of space.

Brighter colors: If the Piranha look a little brighter than normal, it’s a sign of the extreme emotions, either very happy or very angry. You will probably soon find out which.

Dull color: Could be a sign they are getting ready to breed if it suddenly happens. Piranha slightly lose their color during the night anyway.

Pairing: As well as the dulling colors, when Piranha are getting ready to breed the male and female will get side by side and wag tails.

Fin nipping: Often the first warning of stress in the tank, especially if the Piranha are well fed. This is a desist behaviour from the Piranha. Often if one is trying to dominate then this may be resistance to that behaviour Check your Piranha are not stressed.

Hunger strike: Often when Piranha are introduced to a tank they don’t eat for a while. This is normal, or if something else is introduced to the environment. Give them time, eventually hunger will get the better of them.

Skittish: Young Piranha fry are skittish, as is their nature, but adult Piranha can be just as skittish or startled. They can be resting and a noise or light will trigger one of them, which ‘wakes’ the school. Remember, they are naturally shy and timid.

Hiding: Occasionally, Piranha will hide in the decoration and it looks like they are trying to avoid the tank. It could be a nervous fish, but it could be that they are stressed. Check the normal reasons but if they are seeking dark cover, consider that the lights may be too bright. Piranha don’t have eyelids so can’t block out strong light, which is uncomfortable for them.

piranha behavior in a tank


Piranha are a fascinating fish to keep, full of their foibles and behaviours. Some are perfectly naturally and just the way the species behaves.

A lot of problems can be put down to stressed fish, especially if they are combative. The main reasons for stress are inadequate tank size causing confinement, poor water quality, noisy environment, poor overhead cover or excessive lighting.

Any one of those is quite easy to rectify. Stressed fish will undoubtedly lead to unhealthy fish and shorten their lifespan.


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