It is a small species, largely herbivorous with some omnivorous traits, that prefers to live in schools and is partial to shallow waters, especially in blackwater habitats.

Much of I. geisleri's behavior is known from observation in aquaria, but the middlingly-limited research in wild settings lines up with what fish keepers have noted.

There are 20–25 anal-fin rays, fewer than other species, and the dorsal-fin origin is located in the middle of the body, which is unique to I. geisleri within Iguanodectes as a genus.

[8] I. geisleri has retained its original name since description,[9] but has nonetheless changed taxonomic status; in 2011, researchers determined that the genus Iguanodectes, alongside Bryconops and Piabucus, belonged to a new family, Iguanodectidae, as opposed to their previous placement incertae sedis in Characidae.

The rio Negro specifically gets its name because it is a blackwater river, in which microbial decay releases tannins from leaf litter into the water.

[17] I. geisleri is largely in line with this pattern, including leaf litter, fallen fruit, and filamentous algae in its diet,[4] but aquarists have reported omnivorous habits in captive specimens.

[2][7] Aquarists also note that a captive I. geisleri does best if kept in a school ranging from 5 to 10 fish, which remains true of wild counterparts.

Nonetheless, they are exported from their natural habitat, including to locations such as Germany and Poland, and are known to be peaceful fish, which makes them ideal members of a community tank.

Nonetheless, the freshwater inland habitats of South America are under constant pressure from anthropogenic sources, including infrastructure development like hydroelectric dams and chemical disruption from illegal mining operations.