The Dusky Narrow Hatchetfish is native to river channels of much of the Amazon Basin, including in Ecuador, Bolivia, Peru, Venezuela and Brazil.

The hatchet shape with deep keel has powerful chest muscles which allow the fish to leap well out of the water and glide several metres, both to evade predators and also, especially in flooded forests, to pick fruit, seeds and insects/spiders from the trees or to catch insects in flight. It also feeds on those which fall to the water surface.

Most, if not all, of the Triportheus species look very similar to the Dusky Narrow Hatchetfish above. Most also follow their Latin epiphet, e.g. angulatus above is sharply angled, elongatus (not shown) is slimmer and elongated. Looking at those above, most show the typical hatchet shape with a flat top, deep keel and narrow caudal "wrist". Four are slightly more "fish-shaped" (the first one and the last three). These may actually be rotundatus.

Triportheus rotundatus has a variety of common names varying with the source - the collection holding these calls them "Flagtail Butterfly Tetra" but some call them, as in their Spanish name, Black-finned Triportheus, and they have other, disputed common names.

The Flagtail or Black-finned Triportheus is native to slow-moving waterways of some of the Amazon Basin and coastal drainages of the Guianas. It can reach 27cm long but is generally much smaller.