The Killer Instinct: Predatory Fish (and How to Keep Them)

Author: Neale Monks

A fearless fishkeeper clears up the caretaking myths for some of the fiercest predators for the aquarium.

What Is a Predatory Fish?

Predatory fish attract and fascinate many aquarists with their combination of strength, speed, and stealth. But the popularity of predatory fishes doesn’t necessarily mean they’re all that well understood. They are actually a group of fish that are, more often than not, maintained the wrong way, given the wrong food, and kept alongside the wrong tankmates.

One problem is defining what a predatory fish is. Most fish are opportunistically predatory and will consume smaller fish when given the chance. Angelfish are the most infamous example, being more than capable of eating tankmates as large as neon tetras. Other examples of opportunistic predators sold as community fish include archerfish, glassfish, African butterflyfish, spiny eels, and Aplocheilus killifish.

For our purposes, though, a true predatory fish is one that—in the wild at least—would feed primarily on smaller fish. Note that this doesn’t mean they must eat small fish in captivity. In fact, careless use of feeder fish can cause a variety of health problems. The belief that predatory fish need to be fed live fish is emblematic of the misconceptions that surround them.

Predatory Characins

The order Characiformes includes numerous predatory fish species kept by aquarists, most famously the piranhas. Aquarists will encounter others as well, including the bucktooth tetra Exodon paradoxus and the hujeta gar Ctenolucius hujeta.


The piranhas are members of the subfamily Serrasalminae, a group that also includes silver dollars and pacus. Contrary to popular myth, most piranhas school only when young and become territorial loners once mature. The exceptions are found mostly within the genus Pygocentrus, but even these fish need to be kept in a large group if bullying between specimens is to be avoided.

Do piranhas make good pets? They aren’t especially difficult to keep, but then again, they don’t do much either—they mostly drift about looking bored. If you want something entertaining, then bucktooth tetras might be a much better bet.

Exodon paradoxus

Bucktooth tetras reach a maximum length of only 15 cm (6 inches), which makes them easy to house in large groups. This is important because they are notorious bullies. Keep bucktooth tetras in groups of 10 or more, or they’ll bully one another as they jostle for position in the pecking order. Eventually weaker specimens are killed, and it’s not uncommon for aquarists to end up with just a single big specimen. They are equally unpleasant toward other fish and must be kept in a single-species aquarium.

Despite their mean personalities, bucktooth tetras are gorgeous fish attractively marked with red and black on a shimmering golden-green body. They are very active, and a school of these fish swimming about in a planted aquarium is absolutely stunning. Like piranhas they exhibit a feeding-frenzy behavior. They’ll eat just about anything, even flake foods, but dart about manically as they feed, putting on a terrific show. For the aquarist who wants a fish that is both pretty and vicious, bucktooth tetras are hard to beat!

Ctenolucius hujeta

The hujeta gar Ctenolucius hujeta is the most widely traded of the gar-like characins, though related species of Boulengerella and Ctenolucius turn up periodically as well. The hujeta gar grows to a maximum length of about 25 cm (10 inches) and, while every bit a predator, is a rather peaceful fish that works well alongside tankmates of similar size and personality.

In common with most gar-shaped fish, these are stealth predators. Wild fish simply lurk in the shadows waiting for something edible to swim by. If you look at their body shape, you’ll see several adaptations to this mode of life. They have big eyes ideally suited to hunting at dusk and dawn. Their dorsal, anal, and tail fins are large and positioned close together, giving the hujeta gar the ability to accelerate rapidly toward its prey. They also have long, thin jaws filled with sharp teeth that can be opened widely, but being thin, produce little drag as the fish swipes its head sideways toward its prey.

Hujeta gars are easy to keep provided a few basics are taken into consideration. Water quality should be good, and they appreciate a reasonably strong water current. They like to hide underneath floating plants and will be less nervous and likely to jump if kept in tanks with plenty of shade. Hujeta gars are gregarious fish that need to be kept in groups of two or more specimens. They don’t get along well with nippy or aggressive fish, so choose tankmates with care.

Social Behavior

As our survey of the predatory characins has indicated, predatory fish exhibit a range of social behaviors. That said, the majority tend to be comparatively peaceful fish. The explanation is simple enough: If you’re going to creep up on potential prey, there’s no point fighting with rivals and drawing attention to yourself.

Those predatory fish that rely on stealth prefer to be kept in a quiet tank with lots of plants, particularly floating plants, where they can lurk in peace. The hujeta gar is one such example, but others of this type include African leaf fish Ctenopoma acutirostre, needlefish Xenentodon cancila, and the various North American gars Atractosteus and Lepisosteus spp.


Schooling behavior is common but far from ubiquitous. Oddly enough, gar-shaped fish are very often schooling fish, and hujeta gar, needlefish, and the North American gars are all best kept in twos, threes, or larger groups. Leaf-shaped ambush predators like the African leaf fish, South American leaf fish Monocirrhus polyacanthus, and Asian leaf fishes Nandus spp. tend to be more territorial, though rarely aggressive. Provided each has its own hiding place, these predatory fish can work rather well in groups.

Unfortunately there aren’t any reliable rules of thumb when it comes to the laterally compressed, open-water species. As mentioned before, piranhas of the genus Pygocentrus tend to form reasonably stable groups under aquarium conditions, as do bucktooth tetras. But most of the other piranhas, including Pygopristis, Pristobrycon, and Serrasalmus spp., tend to be more or less solitary fish once mature, even if they school when young.

Predators in Community Tanks

Can predatory fish be kept in community tanks? Obviously smaller fish will be viewed as food, but tankmates of similar or larger size might be options. African leaf fish, for example, make good companions for Congo tetras and medium-sized barbs. Similarly, hujeta gar could be combined with silver dollars and medium-sized, non-aggressive cichlids such as festivums and keyholes.

Paradoxically perhaps, those adaptations that make predatory fish skilled hunters often make them poor fighters. Hujeta gar, for example, may have exceptional acceleration but they aren’t agile, and when housed with territorial cichlids often end up with damaged jaws and shredded fins—or worse.

Avoiding Feeder Fish

Surely feeder fish are the perfect diet for predatory fish? Ethics aside, the problem isn’t that they’re live fish, but that they’re feeder fish. The feeder fish species used are normally goldfish Carassius auratus and rosy-red minnows Pimephales promelas. Both of these are members of the family Cyprinidae and, in common with other members of this family, contain large quantities of fat and an enzyme called thiaminase.

Fatty foods cause problems in fish just as they do for humans, and autopsies of predatory fish that have died prematurely have shown that such fish commonly have abnormal deposits of fat around their internal organs. Whether this causes mortality directly or makes them more prone to other diseases isn’t clear, but there certainly seems to be a link between a fat-rich diet and premature death.


Thiaminase is an enzyme that breaks down vitamin B1, also known as thiamin. Fish that eat too much food containing thiaminase suffer from vitamin B deficiency, and that in turn makes them vulnerable to a range of problems. Symptoms of vitamin B deficiency include skin hemorrhages, abdominal swelling, poor growth, abnormal swimming behavior, nervousness, and an overall lack of vigor and resistance to disease. Clearly this is something to avoid.

Feeder Fish Issues

As if these issues weren’t bad enough, fish bred for use as feeders are invariably maintained under squalid conditions. They commonly carry parasites, so even if used infrequently enough to avoid problems with fat and thiaminase, there’s still a good chance your predatory fish will be exposed to some type of parasitic infection.

The bottom line is that neither goldfish nor rosy-red minnows have any place whatsoever in the diet of predatory fish. There is absolutely no justification for the use of them, especially given the wide variety of alternatives that will be mentioned shortly.

But can live fish ever be a safe food? Yes, but this does assume you’re willing to breed them yourself. Livebearers such as mosquitofish and mollies are examples of safe feeder fish that lack thiaminase and are low in fat. They are easy to breed, and prior to use can be gut-loaded with vitamin-rich flake food to ensure optimal nutritional value. Cichlids such as convicts may be useful for the same reasons, but since cichlids have spiny fins, they can choke some predatory fish that are not used to them. If in doubt, don’t use them.

Alternative Live Foods

An adult hujeta gar will easily get through two or three mosquitofish a day, and producing sufficient numbers of live fish to maintain such a predator will be a major problem for most aquarists. The advantage to live foods is that they elicit feeding behaviors, but that doesn’t mean you have to use fish. Various invertebrates work just as well, and if anything, they’re cheaper and more nutritious anyway.


Earthworms are taken by almost all predatory fish—that’s why anglers use them as bait! If you can’t find earthworms in your garden, you can buy them inexpensively from bait shops. But while earthworms are nutritious and unlikely to carry parasites that could harm tropical fish, they are not entirely risk-free. In places where pesticide and herbicidal sprays are used, they could carry these in the soil within their guts and harm your fish. Do not collect earthworms from gardens where such chemicals are used.

River Shrimps

River shrimps are another good choice, though they will need to be used in moderation. While unlikely to contain parasites, river shrimps may contain high levels of thiaminase and, if used excessively, could cause a vitamin B1 deficiency in your fish. Still, they’re a superb food for settling in newly purchased predatory fish not yet ready to accept dead or frozen foods.


Insects are readily taken by many predatory fish, and it’s worth experimenting with things like houseflies, maggots, mealworms, and crickets. Juvenile predatory fish have even more varied tastes and can be expected to consume such things as live brine shrimp, daphnia, mosquito larvae, and bloodworms.

Offer a Variety

Whatever live foods you use, it’s important to ensure you offer a variety. No single live food species will be nutritionally complete. Gut-loading live foods will help a great deal. All that means is that the live food is fed some good-quality flake or pellets before use so that when the predator eats the shrimp or other food, it also eats some flakes or pellets.

Weaning onto Non-Living Foods

Live foods are expensive and difficult to store, and most aquarists prefer to wean their predatory fish onto non-living foods. This is generally not difficult, but it may require patience. The trick is to choose the right foods, offer them in an enticing manner, and let your predatory fish go hungry for a couple of days if it doesn’t show sufficient interest.


Let’s start with choosing which foods to use. You can pick up a good selection from most grocery stores, but one thing to bear in mind is which species contain thiaminase. Shellfish often contain thiaminase, including mussels, prawns, shrimps, scallops, and most types of clams. Certain types of fish also contain thiaminase. The cyprinids have already been mentioned, but thiaminase is also found in mullets, catfish, herrings, sardines, and tuna.

Thiaminase is destroyed by cooking, as are many nutrients. Thus, while meat from the thiaminase species is safe if cooked, raw meat from other species is a better choice.

While any of these might be used once or twice a week, the bulk of your predatory fish’s diet should come from species that do not contain thiaminase. Among the foods that don’t contain it are freshwater perches such as tilapia and sunfish, rainbow trout, Atlantic salmon, smelt, flounders, eel, cod, pollack, Atlantic mackerel, and cockles.


Your local aquarium shop may sell frozen lancefish, typically species of smelt, that can be defrosted as required. These are a very economical way to maintain predatory fish, and because they’re whole fish rather than fillets, they are very nutritious as well. Bigger predators will take them whole, or you can chop them up to feed smaller predators—skin, guts, bones, and all.


Variety is important. Don’t rely on a single type of food; aim to use at least three different types during the week so any nutritional shortcomings in one are balanced by the others. Moreover, predators are more likely to go off their food if they’re bored with it.


So just how do you wean a predatory fish onto non-living foods? Forceps (tweezers) are your friends here, allowing you to hold morsels of food in the water without getting too close to the fish. By wiggling the food about, you can attract the attention of your predatory fish and with luck it’ll strike and swallow the food. To be fair, a newly imported specimen might be a bit hesitant, in which case do not feed the fish for a day or two before trying again.

Forceps also keep you safely away from the jaws of your predator. Few of the species we keep as pets can do serious harm, but things like needlefish and piranhas certainly do have teeth that are sharp enough to draw blood or worse.

Throwing food into the water current is another tried-and-trusted method. The idea here is to make the dead food look alive. Once the predator has lunged at the food item it generally swallows it, so the illusion doesn’t have to last for long. This trick works especially well with needlefish and gar.

Once your predator has learned what’s edible—or more specifically, learned to relate your presence by the aquarium with the appearance of tasty morsels of food—you’ll find you won’t need to trick your predator into eating non-living food anymore. My own hujeta gar learned to take chunks of prawn from my forceps within a day of purchase, and after a week simply snapped up anything thrown into the tank.

Predatory Fish: Gentle Giants?

It’s a shame that predatory fish have become associated with a certain sort of fishkeeping that involves the gratuitous use of feeder fish purely for the sport of watching one fish catch and kill another. My experience is that predatory fish are often-gentle and always-interesting fish that deserve to be more widely kept, and I hope I’ve convinced you of that.

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