Hatchetfish, family Gasteropelecidae (Excerpted from: Extreme Characins Part 1: Hatchets, pikes, and other lethal weapons by Neale Monks)    

Hatchetfish are among the most widely traded oddball characins, presumably because they are peaceful and not particularly difficult to keep. They are basically tetra-like in temperament, but what makes them special is their ability to fly. Hatchetfish have massively expanded pectoral fin muscles attached to a very deep sternum, a striking parallel with the similar development of the pectoral muscles and breastbone in birds, which also rely on those muscles for powered flight. Viewed from the side, hatchets have a very distinctive, semi-circular shape quite unlike anything else in the aquarium. If viewed from above, they are just as peculiar, as instead of being short and flat, the pectoral fins are very long and slightly curled, which makes them much more useful as 'wings'. So while there are many fish that can glide if they jump out of the water at speed, not least of all the celebrated marine flying fish, only the hatchets have true, powered flight. By rapidly fluttering their pectoral fins, they can extend their jumps out of the water to cover several meters. For fish that live close to the surface and in open water, this is an excellent defence mechanism as it quickly gets them out of the range of large predatory fish. However, in an aquarium flying is something that can lead to an untimely death dried up on the carpet, so it is important to keep an aquarium containing these species covered at all time.

Hatchets range in size from small species about 2.5 cm/1" in length like Carnegiella myersi up to about 9 cm/3.5" in the case of Thoracocharax securis. None of the hatchets are especially brightly coloured, though they are all attractive enough in their way. The marble hatchetfish, Carnegiella strigata, is perhaps the standout species in terms of overall prettiness. Although barely 4 cm/1.5" in length, their deep body shape and chocolate brown markings mean that these fish can look very dramatic in the right aquarium. As with hatchetfish generally, open water with a plenty of current is preferred, and while rockwork, ornaments, rooted plants, and other such things are unnecessary, a few floating plants will be appreciated. These fish stay very close to the surface, and any food offered to them should be of the floating variety. Flake will be eaten, but this should be augmented with suitable live or frozen foods, such as bloodworms, mosquito larvae, and wingless fruit flies. Marbled hatchetfish do have a preference for soft and acidic water conditions, but they can be kept successfully in slightly alkaline, moderately hard water provided their other needs are met. As with all the smaller hatchetfish, these fish are skittish and easily bullied, so combine only with peaceful tankmates. Neons, cardinals, pencilfish, and other small, peaceful Amazonian fish would make ideal tankmates.

Typical of the larger hatchetfish is the silver hatchetfish, Gasteropelecus sternicula. The German aquarist Florian Krieger has referred to these delightful animals as 'grey mice', a reference to their tendency to be overlooked in comparison to the jewels of the aquarium. Certainly, these aren't immediately eye-catching fish: they are basically silvery-grey in colour with a blue-grey stripe running from behind the pectoral fins to the base of the tail. But what does make these fish worth keeping is their liveliness. Silver hatchets squabble among themselves almost constantly, and will frequently engage in chases around the tank. Though no harm seems to be done, it's a good idea to make sure you keep more than two specimens so that one fish cannot constantly bully another. In fact all hatchets are best kept in schools of six or more, especially the smaller species. Silver hatchets are fairly robust animals and will adapt well to most conditions. Neutral, not too hard water seems to be best, and as far as tankmates go, pretty much anything not overtly aggressive or nippy will be tolerated. Unlike the smaller hatchetfish, these larger species will swim at all levels of the aquarium, and feeding presents no problems at all: they will eat anything small and meaty, including livebearer fry. Flake is also enjoyed. Silver hatchets appear to have insatiable appetites. They are rather active animals, and it might well be they need a little more food than comparably sized tetras. Healthy, well-fed hatchetfish should have a chunky shape when viewed from the front. Starved specimens often have concave flanks, and may need special care in a quarantine tank before they can be mixed in with ordinary community fish. Otherwise, once settled in, these fish are not particularly difficult to keep.

The spotted hatchetfish, Gasteropelecus maculatus, is one of the biggest species and can reach up to 9 cm/3.5" in length. Again, it is a silvery fish, though perhaps a shade more brilliant than the silver hatchet, and it is immediately recognisable thanks to the pattern of small blue-grey spots on the back half of the body. Given its size and shape, this fish can be very impressive when kept in a decent sized group, and while not as commonly traded as marble and silver hatchets, it is not too difficult to obtain. This species looks superb kept with things like angelfish and discus, but it could equally easily be kept with peaceful gouramis, climbing perch such as Ctenopoma acutirostre, clown loaches, or any other non-aggressive tankmate of suitable size. The spotted hatchetfish needs fairly soft, slightly acidic to neutral water conditions and plenty of oxygen. It is a bit less forgiving than the silver hatchetfish, and good filtration and plenty of water changes seem to be essential to long-term success with this species. A superb fish, to be sure, but one best suited to the more experienced aquarist.

Other fish not belonging to this family are sometimes called hatchetfish, including a great many deep-sea fishes. The only ones of importance to the aquarists are the minnows of the genus Chela, sometimes sold as 'Indian hatchetfish' or 'Asian hatchetfish'. While danio-like in terms of overall care, their body shape is strikingly similar to that of the characin hatchetfish discussed above. Chela caeruleostigmata in particular has the same deep body and extended pectoral fins as its namesakes. Unsurprisingly, these fish are adept jumpers as well, and need to be kept in a securely covered aquarium; in fact, one common name for these fish the 'leaping barb', which underlines nicely just how similar they are to the South American hatchetfish.