Author: Neale Monks
With their stealthy habits and sleek appearances, these miniature pike-like predators are fascinating aquarium subjects—and natural-born killers!
Going by the numbers, piscivorous fish don’t seem like very good choices for the home aquarium.
· Strike one: most are inactive and like nothing better than to swim slowly at the top of the tank or hide away among the plants.
· Strike two: as predators, they need to blend into their surroundings, and this means drab shades of silver, green, or brown rather than the vivacious colors we’re used to with tropical fish.
· Strike three: few will take flake or pellet foods, and weaning them onto foods other than live fish can sometimes be a chore.
But is it really a case of three strikes and you’re out? As ever, it depends on what you want. Neons may have more color, and Corydoras score strongly in the cuteness stakes, but piscivorous fish certainly have a charm of their own. Many have wonderful adaptations that allow them to sneak up on their prey. From toothy maw to powerful tail, these fish combine speed, grace, and power in a way that many aquarists find attractive. The more ecologically minded aquarists will also appreciate the sight of these predatory fishes in communities otherwise containing fish that eat algae or insects. Even if they never eat a live fish in their entire lives, one of these little killers can still play the part of apex predator in the community aquarium, drifting about quietly, like an echo of the red and raw aspects of nature we often forget about.
Predators in the Aquarium
Before we get carried away by this particular call of the wild, we should think a moment about the ethics and practicalities of keeping piscivorous fish. Given the choice, most of these fishes will take live fish over dead or frozen substitutes, but providing a live fish-only diet isn’t always practical, desirable, or even in the best interest of the predatory fish.
Wild Vs. Aquarium
In the wild, predatory fish will be eating a variety of smaller fish, as well as various invertebrates. Adult pike, for example, will certainly eat lots of minnows and small sunfish, but they will also take large insects such as water beetles, shrimps, crabs, and crayfish, or even the occasional aquatic bird or mammal. By varying its diet, the pike will be getting a balance of nutrients it couldn’t get by just eating one type of prey. Keep a pike in an aquarium and feed it goldfish week in and week out, and you’ll end up with a pretty sick pike. Get those goldfish from a pet store where the “feeder goldfish” are kept in unhealthy, overcrowded tanks so they can be sold for a few pennies apiece, and your pike will not only get a nutritionally unbalanced diet, but it’ll end up getting infected with parasites and opportunistic bacteria as well.
Breeding Feeder Fish
The only safe way to use feeder fish is to breed your own. Mollies and guppies are ideal, being easy to breed and nutritious (though often touted as feeder fish, goldfish and rosy red minnows are not at all useful, containing a substance called thiaminase that breaks down Vitamin B1). Guppies and mollies can easily be gut-loaded with algae-rich flakes, which is a key trick for getting essential minerals and vitamins into predatory fish.
Regardless of the practicalities of using feeder fish, there’s an ethical argument too. At this point I’ll lay my cards on the table: most people who use feeder fish do so because they want to, and not because they need to. I can respect that some people enjoy watching one fish catch and eat another, but I don’t, and almost all the predatory fish offered to aquarists can be weaned onto alternatives with a greater or lesser degree of ease. In part, this article is about shattering the myth that predatory fish can only be kept by people willing to use feeder fish. So if you’d like to keep a piscine predator in your aquarium, but don’t want to breed your own livebearers to keep it well fed, then read on!
The Asian Killifish Aplocheilus lineatus
Of all the pocket-sized pikes, the Asian killifish is perhaps the easiest to keep. At around 3½ inches in length, it isn’t especially large, and it can work as a community fish with a minimum of fuss. So while every inch a predator, it is only a threat to things like neon tetras and zebra danios. Gouramis, angelfish, Corydoras, and so on are perfectly safe.
Water and Plants
The Asian killie is very adaptable in terms of water chemistry requirements, inhabiting peaty swamps and slightly brackish streams all along the southern coastline of India. In aquaria, it can be kept successfully in everything from soft and slightly acidic to hard, alkaline, and slightly brackish water. What they do need is an aquarium with lots of plants at the surface, as these fish do tend to jump when startled. Floating plants like Salvinia and hornwort or long-leaved plants such as Cabomba and Vallisneria will inhibit this behavior, but even so, be sure and keep these fish in a covered tank.
Asian killies are sold in two versions, a greenish wild variety and a golden yellow artificial form known as the “golden wonder” killifish. The wild variety is fairly variable in coloration, but it is typically silvery green with a series of thick vertical bands across the back half of the body. Females usually have thicker bands than the males, and the males tend to have a few more yellow spangles on their bodies. Golden wonder killies are silvery yellow around the head and lemon yellow on the body. Some varieties have blue or green spangles on the body or red edges to the fins. Asian killies are very pike-like in their shape and habits, despite their size. They do not like to be overcrowded, and males especially can be territorial.
Though predatory, these fish will eat any small live foods that stay close to the surface. This includes daphnia, brine shrimp, mosquito larvae, fruit flies, and so on. Frozen bloodworms are also enjoyed, though you may want to place these onto the surface one at a time so that the fish can get them easily. If you’re careful, the bloodworms will stay on the surface and the killifish will snap them up eagerly. Most specimens also learn to take flake foods and floating pellets, and a convenient but healthy diet would consist of a mix of both flake and frozen foods.
Pike Cichlids Crenicichla spp.
It would be difficult to discuss aquarium “pikes” without mentioning these popular fish. On the one hand, they come in a variety of sizes, from dwarf species only around 3 inches or so in length (such as Crenicichla compressiceps) to giants that can exceed 10 inches in length (as with the undescribed species sold as Crenicichla sp. “Xingu I”). Most are somewhere between these extremes, with fish around 6 to 8 inches in length being typical.
Broadly speaking, all prefer soft and acidic water conditions and generally get along well with other fish too large to be viewed as food. Single specimens and pairs are usually very aggressive toward conspecifics, however. Some species are shy, and others more outgoing, but a lot depends on the aquarium. In a spacious tank with plenty of plants, these fish will explore their environment a good deal of the time, but in a cramped aquarium—particularly one with few sources of cover—they’ll stay in their chosen cave and only venture out at feeding time, and then only briefly. As such, the dwarf species mentioned should be housed in tanks of at least 30 gallons, while the larger species should be kept in larger tanks suitable for their size.
Pike cichlids can and do eat smaller fish, but like most other cichlids they are quick to learn about the options. River shrimps, earthworms, and other live foods of this type are readily taken from the start, but once settled in these fish will even eat cichlid pellets.
The Pike Livebearer Belonesox belizanus
These ferocious fish may be very closely related to guppies and mollies, but they’re demanding beasts nonetheless. For one thing, they’re much bigger. Female pike livebearers routinely exceed 6 inches in length, with the maximum size recorded being around 8 inches. Males are smaller, just as with mollies and guppies, but the twist here is that female pike livebearers are not averse to eating smaller males if they’re hungry! Describing pike livebearers is easy enough—they look exactly like pikes. Long, greenish body? Check. Big tail fin? Check. Huge jaws crammed with sharp teeth? Check.
Males have a modified anal fin that bends backwards to form a gonopodium used to fertilize the female when they mate. While the fry are not difficult to rear on things like small insects, daphnia, and other suitably sized live foods, they are notoriously cannibalistic. It goes without saying that the parents will eat their offspring, too.
Maintaining pike livebearers is challenging in some ways and easy in others. On the plus side, they are adaptable as far as water chemistry goes. Hard, alkaline water is preferred, but the addition of marine salt mix is strictly optional, though anecdotal evidence suggests it can be helpful under aquarium conditions. The actual salinity used doesn’t matter, as these fish can be adapted to anything up to sea water, but a specific gravity of 1.003 to 1.005 would be about right for most purposes. Marine salt mix increases the pH and elevates the hardness, which will be very helpful if you happen to live in a soft-water area. These fish like their water a little on the warm side, around 26° to 30°C (79° to 86°F). The combination of warm water and a bit of salt seems to nix the problem of fungus, which these fish are particularly prone to, especially if they bump into the glass when startled.
The tricky part of their aquarium care is feeding. Adult wild-caught fish are notoriously difficult to wean onto anything other than live fish. The best alternatives are chunky live foods such as river shrimps and earthworms, and only sometimes have people managed to get these fish regularly eating frozen whitebait and lancefish. Throwing small bits of fish meat into a water current or dangling small frozen fish on cotton thread are two tricks that have been used.
Newborn pike livebearers are much easier and will eat most small aquatic animals including daphnia and insects straightaway, and as they mature it is somewhat easier to train them to accept frozen foods than it is with wild-caught fish. Even so, they remain cannibalistic, and if they decide they don’t like what you’re offering they will turn on one another. All things considered, these are difficult fish that are only suitable for the most dedicated aquarist.
The Needlefish Xenentodon cancila
One of the most extreme of the aquarium “pikes” regularly offered is the freshwater gar, or needlefish, Xenentodon cancila. These are sizeable fish, getting to about 10 to 12 inches in length, but they are best kept in groups, and so will need quite spacious quarters. Needlefish are well named—you will be hard pressed to find any other fish as long and narrow as these!
Their jaws are very slender and perfectly adapted for a sideways lunge at their prey. Oddly enough, while they will happily eat smaller fish, their natural diet consists of crustaceans, and reports to the contrary in many aquarium books are erroneous. Live river shrimps and Gammarus are greedily taken. Small crickets, earthworms, mealworms, and even mosquito larvae are also enjoyed and make good staples. Frozen foods of various types will also be taken, but these do need to move about in the current, and anything that hits the bottom of the tank will be ignored.
As with discussions of their diet, there’s a good deal of unreliable information in the aquarium literature about what water conditions needlefish need in captivity. Many older books suggest these are brackish-water fish, but this may be due to misidentification with other species (most needlefish are indeed marine or brackish-water fish). In fact, these fish normally inhabit fresh water with moderate hardness and a neutral to slightly alkaline pH. While they certainly can be kept in slightly brackish water, doing so isn’t essential to their health.
One thing all aquarists agree upon is that these fish are skittish and easily damaged when they throw themselves about the aquarium in alarm. Quite possibly, the addition of a little salt may inhibit infections should they hurt themselves, but it is better to make sure that the fish don’t get alarmed in the first place. The big tank that you provide for these fish should not have any rocks and bogwood; these fish don’t care about stuff like that. Instead, plant the edges of the tank with tall plants (Vallisneria is ideal) and cover the surface with floating plants such as Salvinia.
Don’t put anything aggressive in the aquarium with the needlefish. If in doubt, leave them by themselves, but otherwise stick with peaceful bottom-dwelling fish such as plecos. Above all, don’t do anything around the tank that would alarm them. Don’t suddenly switch on the aquarium lights when the room is dark, and don’t let nearby doors slam shut noisily. Work quietly when doing tank maintenance and upkeep, and take your time acclimating your needlefish to captive life.
Pike characins are indeed very pike-like in shape, but like the needlefish they like to form schools and mostly hang about in midwater rather than hidden among the plants. Boulengerella lateristriga is the most frequently sold species, most often referred to as the “striped gar.” It reaches 8 to 10 inches in length and is strikingly colored, being basically silvery-brown but with a thick dark band along both flanks and mottled fins. Soft, slightly acidic water is essential, and low nitrate levels seem to help as well. Ctenolucius hujeta, also known variously as the “hujeta,” “rocket gar,” and “blunt-nosed gar,” is another commonly traded species. It is silvery-green with a prominent dark spot on the base of the tail.
Basic care for these fishes is similar to the needlefishes: plenty of swimming space, good water quality, no aggressive tankmates, and floating plants to help them feel secure. Apart from small fish, pike characins will happily take chunky live foods such as mealworms, crickets, and earthworms. They can be weaned onto a frozen food diet fairly readily. They hunt at dusk and dawn, and seem to be attracted by a sudden flash of silver. If you throw a piece of whitebait into a water current at the right time of day, and the aquarium lights turned low, with any luck your pike characins will strike.
Starving a healthy fish for a day or two will do no harm and does help to whet their appetites, but before training these fish in this way make sure you fatten them up on live foods for a week or two beforehand. Newly imported fishes can be very underweight.
Natural Born Killers
Predatory fish, large or small, add an interesting touch to any community tank, and if you don’t want to feed them live fishes you don’t have to, as most will happily accept all sorts of substitutes.
Indeed, a good argument can be made for weaning them onto an alternative diet anyway, not least of which is the risk of transferring parasites from feeder fish to your pet fish. What makes these pocket-sized pikes so special are their subtle, stealthy habits and their sleek appearance. Miniature hunters they might be, but they’re still natural born killers!