Freshwater Oddballs from Around the World

Author: Mark Denaro

Most of us start out in the hobby by keeping a community tank or maybe a few goldfish we won at a fair. For a number of people, that leads to more aquariums. Some of us move on to specialize in a particular family or subfamily of fish, while others may develop an interest in planted or marine aquariums. But for a select few, our interests range into the weird and unusual, the freaky freshwater fish commonly known as oddballs.

When it comes to strange and wonderful fish, there are many to choose from, but which should you try to keep? Let’s take a look at several of the more popular species.

Silver Arowana

The silver arowana (Osteoglossum bicirrhosum) is one of the most popular oddball species. They have the potential to be big fish, which should be considered prior to purchase. They can reach a maximum of 4 feet (122 cm) or more in length, but most fish in captivity grow to about 24 to 30 inches (61 to 76 cm) in length. This still makes silver arowanas big fish, though, and they really should not be kept in tanks of less than 6 feet (183 cm) in length and 2 feet (61 cm) in width.

They’re usually purchased small, so they can be started in smaller tanks, but plans should definitely be in place to move them into larger tanks. And these should be serious plans. Many people start big fish out in small tanks when they’re young with a pipe dream of moving them into a bigger tank at an undetermined time that never comes. This is not fair to the fish, so please don’t do it. If a bigger tank is not realistic for you, then choose a smaller-growing species.


Now, assuming that you can house this species properly, the most important step in keeping it alive is to have a tight, heavy cover. Leave no space open or it will find its way to the floor. These fish leap to catch food in the wild, a habit that does not serve them well in the aquarium. Both captive-bred and wild fish are available in the trade, and both eat readily. Live fish can be used to get them to start feeding in your tank but should not be a staple food, as this will inevitably result in exposing your predator to diseases. Most specimens will readily accept floating foods. Feeding a variety of prepared items should result in a more well-rounded diet for your fish. The landing barge-like mouth means that tankmates should be chosen with care. Arowanas are capable of eating larger fish than the hobbyist may realize.

Water Conditions

Water conditions are not critical for this species, but very efficient filtration and water changes are. Water changes of 25 to 33 percent weekly are appropriate. The fish will grow faster and get larger with more frequent water changes.

African Butterflyfish

Do you really like arowanas but don’t have the tank space to house one? Then consider the African butterflyfish (Pantodon buchholzi). A member of the order Osteoglossiformes like the arowana, P. buchholzi is the only member of the family Pantodontidae. It shares with the arowana a surface-oriented existence, a mouth like a landing barge, and other characteristics, but it does so in a 5-inch (12.5-cm) bundle.

Tank Requirements

Tank height is not important for this species, but it will appreciate a tank with a width of 18 inches (46 cm); any of the “breeder”-style tanks will work, as will most frag tanks, which give you 18 to 24 inches (46 to 61 cm) front to back and usually 12 inches (30 cm) or less in height. African butterflies like to hang out under or near floating vegetation and will enjoy the addition of Nymphaea lily species that are allowed to produce floating pads. Water sprite and other floating plants can also be used to good effect.

This species is a mouthbrooder and can be bred in the aquarium. Floating foods are generally taken, and any fish small enough to be eaten will be. It can be kept in a community aquarium with fish of a similar or larger size so that they are not in danger of being eaten by the African butterfly. They tend to be more active in the evening, and dim lighting will help to make them more active.

Rope Fish

Let’s stay in Africa for the time being to consider our next two species. The rope fish (Erpetoichthys calabaricus) is a member of the bichir family Polypteridae. Ancestral relatives of the rope fish have been found that date back more than 200 million years to the Triassic period. Its swim bladder is divided into two parts, one of which is modified into an accessory breathing organ that allows it to survive in oxygen-poor conditions. Different references provide a wide range of maximum sizes, from 15 to 36 inches (38 to 91 cm). I will say that I have never personally seen one that was more than about 15 inches (38 cm) in length, but they could well grow much larger in the wild.


Rope fish are primarily nocturnal hunters with poor eyesight that rely on their sense of smell. In an aquarium, they should be fed live and frozen foods, with worms and insect larvae given priority. Some specimens will accept sinking pellets, but others will not. Small tankmates will also be eaten, so keep them with fish that cannot fit in their mouths.

Tank Requirements

They will be more active in a dimly lit aquarium but will spend much of their time hiding if the lighting is bright. They are highly sinuous and can turn in a very tight radius, but I still prefer them to be housed in tanks that are at least 18 inches (46 cm) front to back and at least 36 inches (91 cm) in length. A tight-fitting cover is a necessity, as they are adventurous and will leave the aquarium to explore.

Upside-Down Catfish

The upside-down catfish (Synodontis nigriventris) is a good tankmate for the rope fish. It also works well in a lot of community aquariums, so it can be a really good starting point for those who want to begin keeping oddball fish. Growing to a maximum of 4 inches (10 cm), this species can be a good tankmate for anything but the smallest of nonaggressive fish.

Its habit of swimming upside-down is delightful, but it does mean a few things must be considered. This is a nocturnal species that will hide during the day. When doing so, it likes to keep its vulnerable stomach against a surface, so it tends to hide under things. Floating plants are great hiding places for them, as are rocks and wood that form caves.

Upside-down cats are gregarious. They’re more likely to thrive and venture out during the day if kept in a group. Dim lighting will also encourage them to be out more in the daytime. Feeding is not problematic, as they readily accept all prepared and live foods in the average hobbyist’s repertoire.

Tire Track Eel

Our next candidate is the only Asian species included. The tire track eel (Mastacembelus armatus) is regularly available in the trade and is an easy species for hobbyists to obtain. Their habit of hanging out in the substrate with just their head sticking out is very appealing.

Tire track eels are frequently available at about 4 inches (10 cm) in length, making them appear suitable for most aquariums. However, this species can grow up to 3 feet (nearly 1 meter) in length, making them unsuitable for most tanks.

Tank Requirements

When keeping this fish, the aquarium should be as large as possible. The substrate should be soft, and there should be some hiding places in the form of rocks, driftwood, and/or lengths of PVC pipe. Dimly lit tanks are best to keep the fish more active and out of hiding. Feeding presents no difficulties, as most individuals will eat just about anything, including live and dry foods. However, floating foods are usually not accepted; the tire track eel likes to eat prepared foods that are resting on the substrate.

Sharp dÉcor and/or substrates should be avoided, as the eel’s skin is easily damaged and prone to secondary fungal infections. Indeed, the leading causes of death in tire track eels are escape from the aquarium and fungus. Tankmates should grow large enough that the eel poses no threat but be peaceful enough that they pose no threat to the eel.

Pike Livebearer

Let’s cross the ocean to Central America, where we find the pike livebearer or pike top minnow (Belonesox belizanus), which also has an introduced breeding population in Florida. Pike livebearers are best maintained in a species tank or with mild-mannered cichlids that they can’t eat and that will in turn not eat them.


While some may never accept prepared foods, I’ve had pretty good success getting them to eat floating pellets, but understand that you may get some that only eat live fish. They are voracious feeders, so providing enough healthy live food can be problematic. Males typically grow to 4 inches (10 cm), and females usually reach 6 inches (15 cm). They are cannibalistic from the time they are born, which could explain why we generally see more females than males.

Tank Requirements

Pike livebearers need plenty of hiding places, especially if you are trying to raise fry. Dense groupings of bushy plants seem to work well. Because of their size at maturity, I prefer to keep them in tanks of at least 18 by 36 inches (46 by 91 cm). As with several other species discussed here, they are excellent jumpers, so the tank cover should be tight.

Shovelnose Catfish

Our travels now turn south to finish our world tour where we started, in South America. This next species just might be my favorite of this group. The shovelnose catfish (Sorubim lima) is well suited to tanks in the 40- to 75-gallon (151- to 284-liter) range and can consequently be a good choice for most hobbyists.

While capable of reaching 12 inches (30 cm) in length, most individuals will not exceed 8 inches (20 cm) in the aquarium. They are highly predatory and will eat anything that fits in their mouths, so be sure that their tankmates are not too small. The shovelnose catfish will pick out a cave to hide in and will mostly come out at night; dim lighting will increase its daytime appearances.

Live fish may be required as a first food, but I’ve always had great success getting them to take frozen foods and sinking pellets. Angelfish, eartheaters, and other medium-sized peaceful cichlids are good tankmates, as are larger shoaling fishes.

Marbled Headstander

This next species may seem to be a good candidate for a community tank with medium-sized fishes, and it may well be, but there are some aspects of its behavior that must be considered. The marbled headstander (Abramites hypselonotus) is truly an oddball species in both shape and manner of swimming. It spends almost all of its time swimming with its head down.

Its tiny, pointed head contrasts with a fairly sturdy body capable of reaching 7 inches (17.5 cm) in length. This species has the unfortunate habit of being a fin nipper, so it is not suitable for many community tanks. The marbled headstander is more likely to pick on others of its own kind, though, so a large group of other medium to large tetras and active shoaling species is definitely beneficial. Anything solitary and sedate will be nipped.

I’ve watched this species spawn in community tanks, and both wild and tank-raised fish are available in the trade. Spawning was always precipitated by a large water change and the addition of a product that contained tannins, with eggs randomly scattered among live or artificial plants. I generally maintain this species in tanks with dimensions of at least 18 by 36 inches (46 by 91 cm). Aquariums that are well planted along the edges but still have plenty of open swimming space in the front work very well.

Motoro Stingray

The motoro stingray (Potamotrygon motoro) is probably the hardiest of the freshwater stingrays. Unfortunately, it is not one of the smallest—it can grow to about 36 inches (91 cm) across the disc. It is a livebearer, and captive-bred individuals are occasionally available. Most of the fish that reach the hobby are caught in Peru or Colombia. Young ones can be started out in tanks as small as 18 by 36 inches (46 by 91 cm), but definite plans should be in place for larger quarters.

Tank Requirements

The aquarium should have a soft, fine substrate in which the ray can bury itself. Alternatively, leave the bottom of the tank bare. In the long term, only the largest tanks tend to work, and a better choice is to build an indoor pond that is big enough to allow the adults to move freely.

Sex and Feeding

Many hobbyists choose to maintain one male with several females in the hopes that they will spawn. Males have claspers similar to those of sharks, and their sex is easily determined. Most young individuals will require live foods when they are transitioning to aquarium life. Ghost shrimp are a good choice and should be fed some prepared food (i.e., gut-loaded) just prior to being offered to the stingray to increase their nutritional value. Live worms are also useful, and I prefer to use red wrigglers, also known as tropical redworms. These are easily cultured and are an excellent food source. If you do not have young worms available, the larger ones can be cut into smaller pieces.

Watching stingrays feed is particularly interesting, as they will pounce on their food and trap it with the disc. Over time, most can be weaned onto frozen foods, and some will accept dry foods. As stingrays grow, human-grade shrimp and other seafood can be offered.


Those who have the space and resources to devote to this species will be rewarded with fascinating pets that will come to know their owner and can be hand-fed. Don’t forget why they have the word “sting” in their common name, however. The tail spine should always be treated with respect. Typically a defensive weapon, it is used most often when someone or something steps on the ray, but its presence should never be forgotten by the aquarist.

Make Sure They’re Legal in Your State

Prior to obtaining either a motoro stingray or red-belly piranha, do some research to find out if it is legal to keep it in your state. Brazil has just listed all its native stingrays as endangered species; in theory, this will not affect the trade in species from other countries, but only time will tell how the situation will play out. Piranhas are not as readily available as they used to be, because they are now illegal in California, Florida, and New York, the three states through which most fish are imported. Check your local regulations, and buyer beware!

Red-Belly Piranha

Red-belly piranhas (Pygocentrus nattereri) are probably the most misunderstood fish on the planet. They have been vilified as bloodthirsty, voracious killers by the media, as well as in books, movies, and TV shows. They may have Teddy Roosevelt to blame for their notoriety, as he returned from South America with stories about how vicious they were, and his account may have been the first to reach the popular press.


The reality is that these are primarily fin and scale eaters in the wild. Have you ever noticed that most wild fish from South America come in with nipped fins? Piranhas are one of the primary reasons why. Red-bellies are not a particularly feared fish in South America, and if someone is bitten, it’s probably because they did something foolish, such as butchering a chicken and then washing their hands in a stream with piranhas, at the expense of a fingertip.

Now, this is not to say that piranhas can’t be dangerous. They guard their eggs and fry, and they are particularly aggressive while doing so. And in the dry season, piranhas are sometimes caught in oxbow lakes, where, as the season progresses, they eat everything in sight. With the oxbow lake thus depleted, the slowly starving piranhas will eat anything that moves, which is probably the source of most of the stories about them eating things like cows.


Red-belly piranhas can be fed live foods, but I don’t recommend it. If you’re buying them just to watch them eat, you can feed live fish, but understand that doing so significantly increases the likelihood that they will get sick from a disease carried by the feeders. It also makes it more likely that the piranhas will eat each other. On the other hand, if they are fed a diet of prepared foods, they start to look for them and are less likely to eat each other. A varied diet of prepared foods, some of which contain astaxanthin and carotenoids, will make them much more colorful than if they are fed live foods.

Tank Requirements

Maintaining piranhas in captivity is not difficult. These are schooling fish that reach about 9 inches (23 cm) in the aquarium, though occasionally specimens grow to 12 inches (30 cm) in length, so they do need a fair amount of space. I would figure a minimum of 10 gallons (38 liters) per fish, with 20 gallons (76 liters) per fish being even better. In typical home aquariums, they should not have any tankmates. I have seen interesting displays in public aquariums where piranhas were housed with other species, but those fish typically have nipped fins and sometimes bare patches.

Awesome Oddballs

I hope this article has provided a good basis for starting (or continuing) your exploration of the world’s freaky freshwater fish. Now, go forth and find yourself an interesting oddball!