Order Characiformes
Family Crenuchidae
These are arrow-fast, small bottom-dwelling fish distributed across vast areas of South America, where the water is very soft and the pH is more or less acidic. Their body shape with a well-developed dorsal fin is quite unusual and resembles killifish rather than anything else. Males and females are easy to distinguish. Males are brighter and larger than females, with more developed and colorful dorsal and anal fins.
Many species should be kept only in large aquariums because they are very quarrelsome. Territorial behavior is characteristic of mature males of both species. Having claimed an area 20-30 cm in diameter with a secure shelter (a grotto or a suitable driftwood) in the center, they drive away other males of their species, and during the breeding period, crenuchids do not tolerate any foreign fish in their territory at all.
They live quietly and hide in distant corners of the aquarium when their tankmates show excessive activity. The juveniles also live close to the bottom and have camouflage coloration. Crenuchids are rarely longer than 6 cm, but they need a large aquarium. The aquarium must be heavily planted. When setting up the aquarium, shelters must be provided. Only live food is accepted. Aeration, filtration, and water changes are mandatory. They mature at 6-8 months. They live up to 4 years. dH up to 20°; pH 6-7.5; T 20-26°C.
Subfamily Characidiinae
Genus Elachocharax
— Elachocharax pulcher [Myers, 1927]. Synonym: Elachocharax georgia. Brazil, Amazon and Orinoco rivers. 2 cm. Body color is chocolate brown with vertical irregular whitish bands. Keep in a large group. This beautiful species is ideal for a nano aquarium with a sandy bottom, a few floating plants with long roots, driftwood, and leaf litter on the bottom. They require a gentle current from the filter. They spawn in the roots of floating plants or aquarium plants. pH 5.4-6.8; dH up to 8°; T 24-30°C.
Genus Klausewitzia
— Klausewitzia ritae [Gery, 1965]. Brazil, Peru, Amazon. 3 cm. The aquarium should be densely planted with fine-leaved plants. Keep in a school in a small aquarium with other small species; they wither when kept with larger species. During the breeding season, males are quite territorial; it is recommended to keep more females than males. Regular water changes. They act as cleaners by grazing on algae. pH 5.0-6.0; dH 1-8°; T 23-28°C.
Genus Melanocharacidium
— Melanocharacidium blennioides [Eigenmann, 1909]. Synonym: Characidium blennioides. Brazil, Venezuela. 4 cm. Two dark stripes run across the snout. Has brightly colored pelvic fins. Females are slightly larger. Aquarium with plenty of hides and a fast water flow. Hides at the slightest sign of danger. Males are territorial, so the aquarium should have territorial boundaries as well as visual barriers. Takes food only from the bottom.
Genus Odontocharacidium
— Odontocharacidium aphanes [Weitzman & Kanazawa, 1977]. Synonym: Klausewitzia aphanes. (English: Green Dwarf Tetra). Brazil, Colombia, Peru. 2 cm. There is a dark blotch on each lobe of the caudal fin, and eight dark stripes on the back from the nape to the caudal peduncle. Females have fuller bellies, while males have slightly sharper and longer pelvic and dorsal fins.
They are very cautious swimmers who stay mostly among plants, but when necessary, dart out lightning-fast. Males are very aggressive towards each other, so there should be plenty of hiding places in the aquarium, as well as rocks and driftwood to divide territories. They occupy the middle water layer. Young fish grow slowly. pH 5.8-7.0; dH 12°; T 24-28°C.
Genus Crenuchus, Genus Poecilocharax.
Genus Chilodus, Genus Caenotropus. 
