The Xenentodon Needlefishes are some of the most fascinating aquarium inhabitants and are among the easiest freshwater predators to keep. Specimens are more or less fully grown at one foot in length, but the thinness of their bodies means that they produce far less waste than similarly shaped predators like gar (Lepisosteus) or pike cichlids (Crenicichla). They still require relatively large aquariums, especially given their general skittishness, but there are also relatively few restrictions on their potential tankmates, provided they aren’t so small as to fit in a needlefishes surprisingly large mouth.
Two species of Xenentodon exist, which exhibit the familiar geminate pattern of speciation in the Indo-Pacific. On the Indian Ocean side, we find X. cancila distributed throughout Pakistan, India, Bangladesh and east to the Irrawaddy River drainage of Myanmar, while in the Pacific the nearly identical X. canciloides replaces it in the remainder of Southeast Asia, including the larger islands of Indonesia (Sumatra, Borneo, Java?).
References, both aquarium and scientific, have regularly referred to these fishes as occurring in brackish and fresh waters, but evidence of Xenentodon actually occurring in brackish conditions seems virtually non-existent and is argued against by its absence outside of continental Asia. Rather, studies consistently argue that it is found exclusively in freshwaters (including streams, lakes, and swamps), and it is well-established that this species DOES NOT REQUIRE SALT IN CAPTIVITY. Really can’t emphasize that enough. The family it comes from, the Belonidae, is dominated by marine species, so it’s likely that Xenentodonis more salt-tolerant than most, but freshwaters have been invaded independently on at least three separate occasions in this group, including twice in South America.
The differences between the two species are slight and, in general, poorly studied. In a study on the fishes of Borneo, Roberts 1987 reported that the Pacific species grew a bit larger and had proportionately longer jaws with more teeth, but it’s unknown whether this is a genuine difference or simply a result of him studying specimens at different ages. In fact, it remains to be determined if there are even two distinct species here or simply one widespread fish found across all of tropical South Asia.
Studies have shown the natural diet consists of a roughly even mix of small fishes and various insects (especially dragonflies & damselflies, maylies, and surface dwelling fly larvae like mosquitoes). Aquarium specimens will accept all sorts of frozen and live foods, such as bloodworms, chopped earthworms, shrimps, and small fishes. They are gluttonous creatures and should have their rations proportioned appropriately. Obesity does not suit these svelte fishes.
Though their barracuda-like appearance might suggest these would be devilish fishes, they are actually remarkably peaceful. They can be kept in groups or singly, and they will ignore all but the smallest of fishes. In turn, most fishes are apt to ignore these surface-dwellers, making them an ideal choice for a semi-aggressive community aquarium. Just be sure to have a tightly fitting lid, or you may soon find that these relatives of the flyingfish are more than capable of taking flight themselves.