One Tough Tetra: Exodon paradoxus

Author: Seth Gibson

The bucktooth tetra may not look like a piranha, but in the author’s experience, it is more aggressive than one! Learn how to care for these beautiful but vicious animals for a truly fascinating tank in your own home.

The bucktooth tetra (Exodon paradoxus) is an interesting and unique addition to the home aquarium, but it brings along various challenges that must be met for its successful keeping. Most of these difficulties revolve around its nasty behavior in captivity. In fact, some aquarists may argue that, ounce for ounce, E. paradoxus is one of the most aggressive fish available in the hobby.



The bucktooth tetra is native to the Amazon River Basin and Guyana areas. When one hears the name, it brings about images of large, protruding teeth, but in reality, the appearance of E. paradoxus is not that extreme. A casual examination of the mouth reveals that E. paradoxus has serrated lips but not the pronounced dentition that one might associate with a name like bucktooth tetra. Though this may initially seem disappointing to hobbyists looking for a fish with large, visible teeth, E. paradoxus has tremendously powerful jaws for its size, and its teeth are more pronounced and well developed in comparison to that of various community tetras.


The color scheme of E. paradoxus is beautiful: a bright, metallic-silver base accompanied by yellow fins with orange and red tips. Throughout the body are casts of yellow, red, and green. There are also two large black spots, one near the middle of the body and the other at the base of the tail. When maintained under optimum water quality and good lighting, the metallic sheen of the body often reflects blue and purple iridescence.


In the wild, E. paradoxus is a shoaling species with carnivorous tendencies. Insects, small fish, shrimp, and other forms of meaty fare make up the bulk of its diet, but the bucktooth tetra is also a well-known lepidophage (scale eater). This specialized form of feeding creates a problem for aquarists, making almost any fish kept with E. paradoxus at risk for injury.

Keeping the Bucktooth Tetra

E. paradoxus offered for sale at most aquatics stores usually measure 2 to 3 inches long, but they are capable of growing to around 6 inches. It is a slow-growing species, but for every inch that the fish puts on in length, a substantial amount of bulk and body mass is acquired. The bucktooth tetra spends most of its time midwater, but all levels of the aquarium are explored when food is added to the tank or the activity of another inhabitant catches its attention.

Aquarium Requirements

Being an extremely active fish, adult specimens must be kept in aquariums that are both long and wide. The minimum size would be a standard 55-gallon aquarium, but, as always, the bigger the better. E. paradoxus is highly adaptable to a wide range of water parameters, but extremes should be avoided. An ideal pH range would be roughly 6.2 to 7.4. Large, frequent water changes are enjoyed, and the bucktooth tetra often becomes even more active after routine maintenance.

Feeding Recommendations

The bucktooth tetra is not a picky eater in captivity and will accept various foods, such as brine shrimp, mysid shrimp, bloodworms, chopped earthworms, beef heart, cut fish fillet, as well as flake and pellet foods to balance out nutrition. I have a few spare tanks in which I breed feeder guppies and gutload them with veggie flakes before offering them to my various smaller predatory species.

My E. paradoxus are remarkably precise and efficient predators, often snatching guppies at the water surface extremely quickly. The only way I can tell that they are actually catching the guppies is from the small lumps in their stomachs. When any type of food hits the water, it sparks a feeding frenzy unparalleled by most other aquarium species.


Tank decor can include pieces of driftwood, rocks, slate, pots, and artificial caves. Driftwood with a root-system-like appearance makes for striking scenery, as a school of E. paradoxus will endlessly zip in and out of the root-like structures throughout the day. Although bucktooth tetras spends most of their time out in the open, hiding places are utilized to take an occasional break from their seemingly endless activity. E. paradoxus show their colors best when maintained with a dark substrate and live plants accompanied by a dark background. Suitable plants include broad-leaved species, such as Amazon swords and Java fern, as well as various grass-like plants, such as Vallisneria species. Floating plants are also appreciated by E. paradoxus for providing shaded areas and creating a more realistic environment.

Strength in Numbers

Various sources of information on the Internet declare that bucktooth tetras must be kept in schools of ten or more to prevent them from killing one another, but I have found this to be a little far-fetched.I have maintained two separate groups of E. paradoxus at different times, one being six individuals, the other being only three individuals. I did not have any deaths from the bucktooth tetras picking on one another, but I believe my success was based on the fact that when I purchased these groups, I made sure that they were all identical in size. I also had many hiding places in the form of driftwood and slate and ample floating plants so the bucktooth tetras could get away from one another if they wished.

If one observes a tank with multiple E. paradoxus at an aquatics store, the smaller individuals usually have subdued coloring and are trying to hide and the largest individuals are trying to chase after anything in their path. That is why I opted for medium-sized individuals that were almost identical in size the two times I purchased bucktooth tetras in groups. Although they do constantly squabble with one another, I never have had problems with major injuries or death.

It is important to realize, though, that larger individuals will often harass smaller ones to the point of death by not allowing the smaller ones to eat and relentlessly chasing them each and every time they emerge from a hiding place. If one does decide to acquire a group of bucktooth tetras, I would still advise having a spare quarantine or hospital tank on hand just in case a situation arises where specimens must be removed due to aggression. This aspect is also particularly important if an aquarist is considering adding E. paradoxus to a tank with other species.

Tankmates: A Tricky Situation

Some aquarists do not necessarily like the idea of devoting an entire medium- to large-sized aquarium to one species of fish, but this is essential for E. paradoxus. I am a cichlid lover at heart, but even the most aggressive Central and South American cichlids that are similarly sized to E. paradoxus often do not fare well in the same quarters. Bucktooth tetras are so aggressive that most cichlids will simply hide or hang near the bottom of the aquarium to avoid crossing paths, and many times when the cichlid decides to enter the middle or top of the water column, E. paradoxus will knock scales off the broad side of the cichlid’s body for a quick snack.

With Cichlids

I had a 75-gallon aquarium at one point with three cichlids in the 4- to 5-inch range, the most aggressive of which was a 4-inch Midas cichlid (Amphilophus citrinellus). The other two cichlids were a pair of Hypsophrys nicaraguensis. Within less than a week of adding a group of three E. paradoxus that were each 3 inches long, the cichlids were hiding in the lower level of the aquarium. The three bucktooth tetras were patrolling the tank as if they owned it, and each time an E. paradoxus would swim near a hiding place, the various cichlids would lunge with flared gills.

Within a couple of weeks, I had decided to remove the two H. nicaraguensis because they were demonstrating breeding behaviors and I did not think that their eggs or fry would stand a chance with the other rough inhabitants. Although they were acting as if they were ready to lay eggs, the H. nicaraguensis still appeared mildly stressed. They were breathing heavier than usual and were also missing some scales, compliments of the three E. paradoxus.

The move left the Midas cichlid alone with the three bucktooth tetras, but it also gave these inhabitants more space. To his credit, the A. citrinellus never backed down. Each time he would emerge from his favorite cave, his fins would stand erect and his gills would flare to threaten the three E. paradoxus. But in reality, the threat posture of fins standing on edge only made a larger target for the super-fast bucktooth tetras to pick off scales. For months, the three E. paradoxus had perfect fins with no signs of bodily injury, but I began to worry when the A. citrinellus reached 7 ½ inches and the E. paradoxus were only about 4 inches.

A. citrinellus and E. paradoxus

At this time, the A. citrinellus was nearly double the length of the three E. paradoxus and had substantially more body mass. I just had the feeling that one good bite from the A. citrinellus could mean disaster. Sure enough, while I was observing the tank, one of the E. paradoxus bit a scale off the A. citrinellus, and as the scale was sinking, the E. paradoxus went after his snack. Only this time, the A. citrinellus followed his dislodged scale toward the bottom of the tank, and as the E. paradoxus swooped in for the scaly snack, the A. citrinellus violently bit off nearly half of the tail of the E. paradoxus. It was at this point that I decided to remove the three bucktooth tetras and give them to a friend who had an open tank. Despite how things turned out, I was amazed at the high level of aggression that the E. paradoxus displayed—they picked on the A. citrinellus all the way up until the point that they were completely outmatched in size and strength, and they were still trying to dislodge the occasional scale!

A Second Attempt

Though I gave away my three E. paradoxus to my friend, I knew I would acquire a group again if the opportunity presented itself. I eventually set up another 75-gallon aquarium and wanted to designate this as a predator tank. I did some research and ran across some seemingly reliable information on the web that suggested that E. paradoxus would get along better with gar-like species that typically stay toward the surface of the water, as they would not be occupying the same levels of the aquarium.


After a little more than a month, my 75-gallon aquarium was stocked with a rocket gar (Ctenolucius hujeta), a red-tailed chalceus (Chalceus macrolepidotus), a common pleco, and two four-line pim cats (Pimelodus blochii). I had the top and bottom levels of the aquarium occupied with inhabitants, and the icing on the cake would be the addition of six E. paradoxus to occupy midwater.

E. paradoxus

The six E. paradoxus were all 3 inches in length, and within roughly two weeks of being added, they had decimated the rocket gar and red-tailed chalceus, which were both nearly 6 inches. The shiny silver scales of both the top-water fishes proved to be too much of a temptation, and I watched as the bucktooth tetras would casually swim underneath the much larger top-water fishes and dart toward their bellies to dislodge scales. The rocket gar and red-tailed chalceus had to be moved to a hospital tank, but the good news was that the common pleco and pim cats were faring much better with the six E. paradoxus.

Pim Cats

The pim cats, which have smooth skin and lack scales, had visible marks on the broad sides of their bodies from the E. paradoxus sampling them in hopes of finding scales. This never appeared to cause any major physical harm or stress, and the pim cats were able to stay in good health. Although I never saw the bucktooth tetras attack the pleco, it occasionally had split fins, which I could only guess was from the six E. paradoxus, but it also remained in good health without any major injury.

Tetras Running the Show

Meanwhile, the friend to whom I had given the previous three E. paradoxus also made an attempt at providing an appropriate tankmate by purchasing a 4½-inch red-bellied piranha (Pygocentrus nattereri). I went to my friend’s house to observe the spectacle, as the three E. paradoxus were relentless with the piranha. The bucktooth tetras forced the piranha into hiding the majority of the time and knocked off scales any chance they could get. The piranha would hide among the densely planted areas of the tank as the three E. paradoxus took center stage, moving about above the planted areas almost as if they were daring the piranha to come out of hiding. There is a good chance that if the piranha substantially outgrows the bucktooth tetras, there will be a similar or worse incident than what I experienced with the same three E. paradoxus and A. citrinellus. But for the time being, the bucktooth tetras are running the show despite the piranha having much more body mass and being slightly longer.

A Truly Aggressive Aquarium

E. paradoxus is a hardy, unique, and ultra-aggressive species with certain challenges that must be met before it can be successfully maintained in an aquarium. The previous examples shed light on the quandary of maintaining bucktooth tetras—it is extremely difficult to find appropriate tankmates. E. paradoxus will most likely pick on and out-compete even the most aggressive tankmates that are similar in size, including most cichlids. Some others can endure the aggression of bucktooth tetras until they are large enough to inflict serious damage or death upon the E. paradoxus, however.

Based on my past experiences, there is not much else that I will attempt to maintain with E. paradoxus other than armored catfishes, such as those of the family Loricariidae, and robust, smooth-skinned catfishes, such those of the Pimelodidae family. Although there are undoubtedly other species that will work as tankmates for bucktooth tetras not covered in this article, one should still have a spare hospital tank on hand, as aggression from one species or another may warrant the removal of a fish to save its life. For any aquarist who can devote an appropriate aquarium to E. paradoxus, I recommend giving this tough tetra try. You won’t be disappointed by its brilliant coloration, ceaseless activity, and unparalleled aggression.

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