What is the Squarest fish in the world?

In this post, we will learn a few of the Square Fish curiosities, the challenges of raising them in an aquarium, as well as their biology and ecology. 

What is the Squarest fish in the world?

The Boxfish (Ostraciidae) is the squarest fish in the world, due to its cubic body shape. Many species of this fish group are yellow and can be found in shallow water. 

The squarest fish in the world

Ostraciides are known as Boxfish or Vault fish because of their morphology, more specifically, because of the bony plates that cover their head and body, giving them the appearance of a box or a vault. These are fish that swim poorly because they do not have ventral fins and, at the same time, they cannot move in water at the same speed as other aquatic beings.

The Coffer fish belongs to the Ostraciidae family, which includes a group of square fish that belong to the order of Tetraodontiformes, related to the Pufferfish species. Its square aspect is due to the fusion between the plate-shaped scales and its solid triangular carapace. Thus, it acquires the appearance of a box from which its tail and small fins protrude. 

Ostraciidae family

They are characterized by having the body enclosed in a bony box composed of hexagonal plates. Some of the species are called Cowfish (with or without “horns”, bony lumps located on the head). The coat of the different species varies from pale green with blue spots and stripes to yellow and brown, with blue, white, or purple spots. 

These animals stand out for their particular appearance, but they also display features unique to the animal kingdom, in addition to their external anatomical features. 

Their preferred habitat is coral reefs in tropical waters. They feed on molluscs, crustaceans, and corals, which they grind with their strong jaws. They can poison the water where they live, being harmful to other fish. They reproduce through microscopic eggs. 

Square fish habitat

The squarest fish in the world live in coral reefs and the tropical sea, at a depth between 1 and 280 meters from the surface. Young boxfish usually are kept in groups, which is very characteristic in coral reef areas. In these environments, they look for places to protect themselves.

The yellow colour of its body in the case of young specimens is an alert of its toxicity to other fish that are around it and, at the same time, a defence mechanism.  

Creating square fish in tanks

If you are interested in having a Boxfish in an aquarium, be careful when placing it in the water. They can attack other fish by expelling toxins. 

You should use a good aquatic decoration similar to its habitat, with excellent lighting and moderate temperature. With the above ideal conditions, the Boxfish can live between 4 and 5 years.

In captivity, the Boxfish can eat small amounts of seaweed, squid, and mussels. In its natural habitat, this fish feeds on small animals such as worms and crustaceans. It is advisable to keep it in a large aquarium, due to the toxin in its body.

In addition, it should be noted that being saltwater, the Boxfish requires extremely specific water parameters. Simulating the salinity, pH, and nutrient flux of the sea in an aquarium is a very difficult task. Therefore, the maintenance of a marine aquarium is reserved for the most experienced aquarists. 

Tank size requirements

When you come across a fish the size of a small caramel in the store, no one would suspect that in a few months it can reach almost 50 cm in length, making its permanence in the absolute majority of residential aquariums unfeasible. 

To make matters worse, it loses much of its youthful charm, going from radiant yellow to pale beige until it reaches its final form: a dull purple with yellow streaks that look more like scars and a pink snout. 

Reef tank incompatibility

A more trivial problem that can cause a lot of headaches to the unsuspecting is the fact that the Boxfish is not “reef safe”.

When it is small, it does not pose any problem. However, when it grows, it may be that your shrimp, soft corals, anemones, and tridacnas will start to disappear one by one, straight into the belly.

It is recommended to place it in a “fish only” aquarium, with peaceful fish of reasonable size. Cleaner fish like Cleander wrasses (labroides dimidiatus) and Neon Gobies (Elacatinus oceans) can represent an endless source of torment for the poor Boxfish, culminating in the release of ostracitoxin. Thus, it is better to avoid them. 

The ideal would be for the Boxfish to have an aquarium just for itself, making monitoring easier. In this context, it can turn into a fascinating pet fish like pufferfish, able to recognize its owner and even do cute things, like splashing water in its face. 

Disease prone

Due to their lack of scales, Square Fish are very prone to diseases such as ich and velvet diseases. Too bad that any medication (such as copper and formalin) can stress the fish to the point of inducing the secretion of poisonous mucus. It is always better to use freshwater baths, foods with natural medicines (garlic and vitamins, for example), and small partial water changes. 

Feeding difficulty

Square fish is omnivorous and keen on a very varied menu, from algae to worms, molluscs, and crustaceans. The youngsters need to eat very often and even then, there is no guarantee that they are getting the necessary nutrition. 

There is also the acclimatization factor in a new tank, which can make the fish go on a fateful hunger strike. Try breaking the fast by offering live food (artemia or ghost shrimp) and then sliced ​​squid, oysters, mussels, Nori seaweed, and Spirulina. 

To make matters even more difficult in a tank, there is also the fact that this fish does not have a lateral line and the square shape is not hydrodynamic making it a very clumsy swimmer, who cannot compete with others. To ensure it is eating well, offer food several times a day.

Square fish is a very complicated animal to be kept in a tank, a danger to itself and other fish, with a very low survival rate in aquariums.


In this post, we learnt a few of the Square Fish curiosities, the challenges of raising them in an aquarium, as well as their biology and ecology.

If you have any thoughts or doubts, feel free to drop us in a comment below!

Frequently Asked Questions (FAQs): Square Fish

Is a Boxfish a fish?

Yes, the Boxfish is a marine fish species of the family Ostraciontidae or Ostraciidae. 

Is the Boxfish poisonous?

Yes, some of the Boxfish species can exude toxins through their mucous glands if stressed.

Are Boxfish and Cowfish the same animal?

Yes, Boxfish or Cowfish are members of the Ostraciidae family. They are bony fish related to Pufferfish.

Can you eat boxfish?

Boxfish are poisonous, they can produce toxins that are harmful to humans.

How many species of Boxfish are there?

There are around 23 species of Boxfish. They are divided into 6 different genera.

What does a Boxfish eat?

The Boxfish usually feeds mainly on algae. However, they can also feed on small fish, molluscs, crustaceans, and sponges.


Hove, J. R., O’Bryan, L. M., Gordon, M. S., Webb, P. W., & Weihs, D. (2001). Boxfishes (Teleostei: Ostraciidae) as a model system for fishes swimming with many fins: kinematics. Journal of Experimental Biology, 204(8), 1459-1471.

Bariche, M. (2011). First record of the cube boxfish Ostracion cubicus (Ostraciidae) and additional records of Champsodon vorax (Champsodontidae) from the Mediterranean. aqua, 17(4), 181-184.

Klassen, G. J. (1995). Phylogeny and biogeography of the Ostraciinae (Tetraodontiformes: Ostraciidae). Bulletin of Marine Science, 57(2), 393-441.