Scheel (1968) observed the gut contents were invariably ants, others have reported insects, worms and aquatic crustaceans.

Aquarium specimens are invariably seen eating protozoans from the water column and the surfaces of leaves, however these are not apparent as stomach contents.

Many members of the family Cyprinodontidae (the pupfishes) eat plant material as well and some have adapted to a diet very high in algae to the point where one, the American Flag Fish, is a renowned algae eater in the aquarium, in spite of belonging to an order of fishes that do not generally consume any plant material.

The largest species is the cuatro ojos (Anableps dowei), which measures 34 cm (13 in)[2] in length, while the smallest, the least killifish (Heterandria formosa), is just 8 mm (0.31 in) long as an adult.

The family Aplocheilidae has been expanded by some authorities to include all the killifishes with three subfamilies, Aplocheilinae, Cynolebiinae and Nothobranchiinae,[4] but this is not the classification adopted in the 5th Edition of Fishes of the World.