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Tropical Fish to Avoid

Some tropical fish species don't belong in your aquarium.

By Jay Hemdal |

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Home aquarists have three major questions facing them when contemplating the purchase of a new freshwater or saltwater fish, or another aquatic animal for their aquariums: Can I afford it? Is this specimen healthy? Will this species thrive given the aquarium conditions I can offer it?

The answer to the first question is perhaps best determined by conferring with your spouse. The second question can be answered if you trust the information given by the dealer, or if the aquarist has a good working knowledge of the relative health of tropical fish and is able to observe the individual fish closely for signs of impending health problems. To correctly answer the third question you must know something about the natural history and captive habits of that particular fish species.

With this final point in mind, I'd like to outline the reasons why some species of tropical fish may not be well suited for the typical home aquarium. Too many times I have heard the following question from aquarists: "I bought this tropical fish two weeks ago and it hasn't eaten any food yet. What should I do?"

The answer is often, "I wish you had asked me this about 15 days ago, because this tropical fish species simply does not survive well in the confines of any home aquarium." Granted, advances in this hobby are clearly not made unless aquarists move ahead toward new challenges, but there's no reason to forge ahead blindly. You will only make the same mistakes that others have made before you. Knowing the potential challenges a difficult specimen may offer is different from simply acquiring a fish and then having to deal with the resulting problems after the fact.

There are seven criteria that an aquarist should understand in order to avoid buying a problem tropical fish species. Follow them, and I guarantee your fishkeeping life will be much easier.

1) The tropical fish species may possess some means by which it can seriously harm an aquarist or family member. Stinging, biting, ingestion and direct skin contact are all ways in which some aquatic fish species can cause physical harm to a human. Taking extra care while handling a venomous or poisonous animal isn't always enough of a precaution.

I've been stung by lionfish three times in 20 years, yet each time I believed I was being very careful in working with them. Luckily for me, I don't seem to be adversely affected by the venom of these fish. Other people have proven to be highly allergic to the sting of a lionfish. In their cases, a similar mistake in handling these animals could have proved life-threatening. There are other species, much more venomous than lionfish, which are routinely offered for sale by your neighborhood pet store. Stonefish are perhaps the best (or should I say worst) example of a species in this category.

2) Some tropical fish have exacting food requirements that are not possible to meet in the normal home aquarium. These fish specimens will generally starve to death once their internal energy reserves are exhausted. Observations such as "I haven't seen it eat yet, but it must be eating something because it has been living in my aquarium for three weeks now" are often proved incorrect after another week or so.

Some tropical fish species, such as moray eels and large butterflyfish, have been known to survive for more than eight months without consuming any food, at which point they must begin feeding or they will invariably perish. Any aquatic organism (aside from filter feeders and those that gain their food energy from symbiotic algae) should show much gusto when they are feeding. If they do not, there will certainly be problems down the road in terms of long-term success.

This is really an easy problem to avoid. Just request that the pet store employee feed your potential fish purchase in your presence before a sale is made. If the animal refuses to feed well during this demonstration, the aquarist is advised to pass on it and select a different specimen. Coral-eating butterflyfish are perhaps the best known example of this type of fish.

3) Although tropical fish specimens in this category will readily accept standard aquarium fare, they may nonetheless waste away and perish within a few months. And, it is always very confusing when it does occur, even though this is a relatively rare problem. The aquarist may simply attribute the demise of the fish to some basic problem with their aquarium's operation, not realizing that while the animal was taking in food, the diet was not properly assimilated and malnutrition was the actual cause of death. The rock beauty angelfish is perhaps the premier example of this type of fish.

4) Some tropical fish species may exhibit characteristics of both of the previous two categories, but not to such an extreme degree. A few of these fish will adapt to long-term aquarium confinement if given special attention, but beginning and intermediate aquarists might wish to avoid them until they gain more experience in aquarium husbandry techniques. The harlequin tuskfish is an example of a saltwater fish that belongs in this category.

5) In some cases, feeding the fish is not the major obstacle, but other requirements, such as water quality or other physical necessities, cause the species to be considered extremely delicate to maintain in captivity. Pelagic jellyfish are examples of animals in this category.

6) There are some species of animals that, while perhaps simple to care for, are considered ecologically threatened or endangered in the country they originate from. For the sake of their threatened natural populations, these animals should not be purchased unless they will somehow contribute to a captive-breeding program established for the ultimate benefit of the species. At the very least, some aquarists may find themselves at odds with United States laws when trying to purchase these animals.

"Sting operations" conducted by government officials in which the person purchasing these illegally collected animals is prosecuted once the purchase is made from the "dealer," who is actually a government agent, are not unknown. To date, this type of situation has occurred mainly with pet reptiles and birds, but it could be expected to expand to aquatic animals in the future. The South American Arapaima is an example of a species whose collection and sale is governed by strict laws, and any purchase of this species should only be made if legitimate importation documents are first shown by the dealer.

7) This group of tropical fish may be quite hardy, but they are capable of quickly outgrowing their aquariums. The pacu is perhaps the best example of a commonly sold fish that is able to outgrow even the largest home aquarium. Often sold in pet stores as babies for less than $5, they are capable of growing to 24 inches in length in a few short years. Public aquariums rarely have the space to accept these outsized fish, so avoiding such a purchase is the best way to circumvent the problem of how to later find a home for a huge adult fish. Do not rely on the small size of your aquarium to stunt the growth of these fish. This will only slow the growth rate, not stop it. The same applies to the abhorrent technique of starving fast-growing fish in order to try to keep them at a smaller size.

The Categories at Work
When describing the general habits of an entire species group, and then applying that information to individuals of that species, there will be variations and exceptions. So, the following tropical fish species accounts describe the expected problems of the majority of the individuals of each species, but not all.

Occasionally, a particular fish is discovered that doesn't fit the norm. A unique specimen of saltwater butterflyfish that normally only eats living coral may take to flake food very readily. This is a fairly rare occurrence. More often the opposite is true — an individual of a species that normally thrives in captivity (and thus is not listed below) simply doesn't do well for some unforeseen reason.

Still, some aquarists may take exception to finding a particular tropical fish species listed in this account of animals to avoid. That they may have had good success with one member of that species in their own aquariums does not mean that other aquarists will. These listings were compiled after much research (and more than a bit of soul-searching), and reflect what the majority of aquarists will experience if they choose to acquire these fish species for their home aquariums.

Freshwater Fishes to Avoid
Arapaima (Pirarucu) — Arapaima gigas: categories 5,6,7

This large South American fish is listed by the International Union for the Conservation of Nature and Natural Resources (IUCN) and by the Convention on International Trade in Exotic Species (CITES) as being a vulnerable species. A difficult-to-acquire export permit is needed to legally purchase these fish. Anyone managing to locate a legally imported Arapaima would soon be dismayed at the speed at which it will rapidly outgrow any home aquarium, as it attains its maximum adult captive length of more than 6 feet.

Asian arowana (Bonytongue) — Scleropages formosus: category 6

Although very similar in terms of aquarium needs to the common silver arowana from South America, this fish is listed as a threatened species, the trade of which is restricted by international treaty. Illegally imported individuals should be avoided.

Black arowana (Aruana, Arawana) — Osteoglossum ferreirai: category 4

Imported as juveniles, the vast majority of these perish within a few weeks because aquarists expect them to have similar husbandry requirements as the more common (and much hardier) silver arowana. Even minimal success with this species seems to require frequent feedings of live wingless fruit flies and pinhead crickets until the fish is at least 6 inches long.

Butterfly "goby" — Scorpaena sp.: category 1

This is not a goby fish at all, but rather a juvenile brackish water scorpionfish that is sometimes sold to unsuspecting freshwater aquarists. The sting from the venomous dorsal spines of even a small specimen have been reported to be extremely painful. One person even required the attention of a physician to treat the injury.

"Freshwater" moray, Gymnothorax sp.: category 2

Commonly offered for sale as "freshwater" fish, these juvenile saltwater eels simply do not accept food when kept in a purely freshwater aquarium. Acclimating them to a saltwater or even brackish aquarium will motivate them to begin feeding. Because moray eels can survive for months without food, many aquarists believe that their animals must be eating something, simply by virtue of how long it has been in their aquariums. Unless moved to a saltwater aquarium, time will eventually prove this assumption wrong.

Freshwater stingray — Potamotrygon sp.: categories 1,7

Most aquarists are well aware of the venomous spines these fish posses on their tails, yet the number of incidents in which aquarists are injured indicates that these fish should be handled extremely cautiously, if at all. Most injuries seem to occur during netting and transfer of the fish. The spine often gets tangled in the net, and the aquarist is stung while trying to free the fish. Some species of freshwater stingrays are capable of growing to a large size, with body diameters in excess of 24 inches.

Giant gourami (Goramy) — Osphronemus gorami: category 7

These sedentary, fairly peaceful fish will grow to a length of more than 24 inches, and thus will easily outgrow all but the largest home aquariums. Previously only rarely offered for sale, recent importations of large numbers of juveniles have resulted in many of these fish being sold to home aquarists who then must deal with them as they grow too large for their aquariums. Another fish (Colisa fasciata) is also sold as the "giant gourami," but rarely exceeds 5 inches in length.

Pacu — Colossoma macropomum (and related species): category 7

As described earlier, these commonly available fish are capable of outgrowing the largest of home aquariums as they attain their maximum size of more than 60 pounds. Some aquarists may joke that a "fish fry" would be in order when their fish mature, and while they are reported to make a tasty meal, a better solution would be to avoid purchasing these fish in the first place.

Paddlefish — Polyodon spathula: categories 4,5,6,7

Although highly endangered in their native habitat of the Mississippi river drainage, paddlefish captively raised by state fisheries departments sometimes make their way into pet stores. Growing to a maximum length of 60 inches and a weight of more than 180 pounds, its size is a major strike against it as a home aquarium fish. These fish also must swim constantly, and have difficulty navigating the confines of even a large aquarium, bruising their long rostrum (snout) in collisions with the aquarium walls, often with fatal results. Feeding these fish is also difficult as they prefer planktonic food, which they sift from the water with their huge mouths. Generally, an automatic feeding device is required to continuously suspend enough food in the water to meet their needs.

Piranha — Pygocentrus and Serrasalmus sp.: category 1

The sale of this fish is restricted in some states because it is quite capable of biting the hand that feeds it. Although a very common fish in many pet stores, one should think seriously before purchasing it. Despite their carnivorous habits, they are fairly boring fish to keep in an aquarium and many people tire of keeping them quickly.

Snakeheads — Channa (= Ophicephalus) sp.: category 7

Prone to jumping out of aquariums, these powerful fish will soon outgrow even a 300-gallon home aquarium. Aggressive toward most other fish, there is really very little about this group to recommend them for home aquarists. One pet store proudly displayed a sub-adult, 24-inch red-line snakehead (Channa micropeltes) in a 55-gallon aquarium at the front of the store. Upon visiting the store a few weeks later, the fish (and aquarium) were gone. According to store personnel, it seems that it "must have tried to turn around in the aquarium" one night and cracked the glass, draining out the water and causing its demise!

Tiger shovelnose catfish — Pseudoplatystoma fasciatum: category 7

There are many South American catfish that reach a size too large for home aquariums, but this species also happens to be highly excitable, often injuring itself by ramming obstacles when confined to an undersized aquarium.

The following freshwater fish are either rarely encountered or are problem fish primarily for beginning aquarists who are not familiar enough with the particular husbandry needs of that species. They are listed by common name, scientific name and category.

African arowana, Heterotis niloticus, 3; Barramundi "perch," Lates sp., 7; biara (hujetas), Rhaphiodon vulpinus, 4, 5; Candiru catfish, Vandellia cirrhosa, 1; Chinese sucker, Myxo~cyprinus asiaticus, 4, 5; chocolate gourami, Sphaerichthys osphro~menoides, 4; darters, Etheostoma and related species, 4, 6; dragonfish (violet goby), Gobioides broussonnetii, 4, 5; electric catfish, Malapterurus electricus, 1; electric eel, Electrophorus electricus, 1, 5, 7; firewood catfish, Sorubimichthys planiceps, 7; payara, Hydrolicus scomberoides, 4, 5; redtail catfish, Phractocephalus hemioliopterus, 7; "shark-cat" (sea catfish), Arius sp., 1, 7 ; sturgeon, Acipenser sp., 3, 7; tigerfish, Hydrocynus goliath, 7.

There are also a fair number of live "aquatic" plants that the aquarist might wish to avoid as they have a history of not thriving under normal home aquarium conditions. The most notorious is the "princess pine," which is merely the cut branches from a species of evergreen tree held together with a rubber band. These are incapable of growing in aquariums, although they will remain green for a month or so.

There are also a number of emergent bog plants that are sold as true aquatic plants (the Brazilian sword is one example). In reality, they only thrive in conditions where their leaves are allowed access to the air. When completely submerged in aquarium water, they survive for varying lengths of time, usually just a month or two.

Generally, any plant that is capable of supporting its own weight when held out of the water by its base is adapted for only a partially aquatic existence. Totally aquatic plants, (the type that do best in aquariums) will flop over to some extent when held upright out of the water.

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Reader Comments

Nick    leesburg, VA

4/17/2012 9:54:01 AM

Good information to know.

Julie    Gilbert, AZ

10/31/2009 9:01:47 AM

Thanks for the useful information.

gobyowner    central coast, CA

7/17/2008 1:12:21 PM

I am curious to know why they violet goby is listed. They are easy to find, easy to care for, peaceful, and sure the can get big, but not unmanagable. Anyway, the best way to learn about a fish is to own it (and sometimes learn the hard way) or to talk to a bunch of people that owned the fish and make the best decision through their experiences.

Brian    Greer, SC

1/25/2008 5:29:25 AM

Great job! If only I had done the things you listed when I was just starting the hobby!! But you live and learn! Thanks for the grat article!

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