Order Characiformes
Family Hemiodontidae
Inhabit the northern part of South America. Fishes of various species in this family remain relatively small. From an aquarium perspective, these genera and species have never played a major role. A systematic feature is the absence of teeth on the lower jaw, which served as the basis for calling them half-toothed. Their mouth is terminal, with sparse maxillary teeth arranged in a single row. The jaws bear at least four relatively movable teeth adapted for catching benthic invertebrates and scraping off algae. Size ranges from 5 to 28 cm. Sexual dimorphism is poorly expressed. Males are brighter and slimmer than females.
Peaceful, schooling fish. All half-toothed fishes can be kept in large aquariums and together with other species. The only thing they need is swimming space. They love the company of their own kind. Keeping them requires a spacious aquarium where open areas alternate with areas planted with hard-leaved plants. A cover glass is mandatory. They are accustomed to soft or very soft, relatively acidic water. Aeration, filtration, and water changes are mandatory. Unpretentious regarding food. They reach sexual maturity at about 2 years of age. They live 3–6 years. dH 5–20°; pH 6–7.8; T 22–28°C.
Genus Hemiodus
Also kept: — Hemiodus semitaeniatus (Kner, 1858). (Halfline Hemiodus). South America, Tapajós, Madeira, Orinoco rivers. 18 cm. Flanks are silvery-gray, shades vary depending on the light. Round black spots in the middle of the body. From this spot, a black stripe runs up to the lower lobe of the caudal fin. Fins are usually colorless, may have a slight reddish tint. Suitable only for large aquariums; — Hemiodus ternetzi (Myers, 1927). Brazil. 10–12 cm. Slender and elongated body shape. Body is silvery, a long black stripe begins behind the eyes and ends at the lower corner of the caudal fin, thickness increases from the gills to the tail. Fins are colorless. Peaceful, very active species.
— Slender Hemiodus — Hemiodus gracilis [Gunther, 1864]. Synonym: Hemiodopsis gracilis. (Slender Hemiodus). South America, from Guyana to Peru, basins of the Amazon, Negro, Tapajós, Madeira, Orinoco rivers. 12–15 cm, in an aquarium about 10 cm. Fusiform, elongated and streamlined body, caudal fin is forked.
The main color of this fish is silvery-gray. Approximately in the middle part of the body, there is a dark spot, from which an anthracite-colored line runs towards the caudal fin, passing onto the lower lobe of the deeply notched forked tail, as if outlining it. On the outer side of the lower lobe of the caudal fin, there are small white lines that turn pink or red as the fish matures. Specimens from Ecuador have black dots on their sides. Males are smaller and brighter.
A large fish that lives in schools. Peaceful, very active, these fish look especially spectacular in a school of 6–8 individuals. Fish are friendly, playful, and look wonderful in interspecific and mixed schools with similar active companions. A nimble, fast swimmer; a spacious aquarium with a free space for swimming, devoid of plants and decorations, is needed. Plants and decorations can be placed along the perimeter of the aquarium at the back and sides. Green plants strongly emphasize the attractiveness of the fish, but it should be noted that the fish do not mind tasting delicate plant leaves, so plants with hard leaves should be planted or some species replaced with artificial ones altogether.
In the wild, they inhabit fast-flowing rapids, so aquariums must be equipped with a powerful filter and aerator. They like to stay in the current, predominantly in the middle water layer. Very jumpy, so a cover glass is a must. The aquarium must be equipped with a high-performance filter and aerator, in addition to weekly water changes. Sharp fluctuations in water parameters can lead to death. Adult fish feed on plankton, insects and their larvae, tubifex (taken reluctantly from the bottom), granulated and flake foods. It is important not to overfeed breeders with dry concentrates, otherwise fatty degeneration of the liver and gonads may occur.
Prンone to stress during transportation, transfer, and acclimatization (about 3 weeks). During this period, special care is needed not to frighten them. After some time, this fish will turn into a completely normal, surprisingly beautiful pet. They mature after two years. They live up to 7 years. pH 6.0–7.0; dH 4–15°; T 23–27°C.
— Barred Hemiodus — Hemiodus quadrimaculatus [Pellegrin, 1909]. Synonym: Hemiodopsis quadrimaculatus. (Barred Hemiodus). Guiana, Kamopi River. 10–14 cm. Streamlined, elongated body, almost fusiform, with small fins. Relatively large eyes.
Bright yellow body with three vertical black stripes and a spot on the caudal peduncle, usually a red adipose fin, other fins pale yellow, caudal fin with black outer rays. Differences between male and female can only be seen at an adult age. The subspecies H. quadrimaculatus vorderwinkleri has one more stripe.
Peaceful, very active, keep in a school of 6–8 individuals. They like to stay in the current. A nimble, fast swimmer that needs plenty of space and a spacious aquarium. Very jumpy, requiring a cover glass and floating plants with long roots. Keeps mainly in the middle layer. Gets along well in large community aquariums with similar active fish. Water should be soft and acidic, with a high oxygen content. Their diet mainly consists of live food and supplementary plant matter. During feeding, they can aggressively chase away other fish. pH 6.0–7.5; T 23–27°C.
— Hemiodus unimaculatus [Bloch, 1794]. Synonym: Anisitsia notata, Hemiodus crenidens, microcephalus, kappleri. Guyana, Guiana, Brazil, in floodplain lakes and fast-flowing river sections. 18 cm. Olive-silvery with a strong silvery sheen, a black spot behind the dorsal fin, and red- or orange-black tail lobes. This species resembles Hemiodus orthonops.
Very active, keep in a school of 6–8 individuals. A nimble, fast swimmer, they need plenty of space, a spacious aquarium with floating plants on the surface. Very jumpy, so it must be covered with a glass lid. Keeps mainly in the middle layer. Gets along well in large community aquariums with similar active companions.
Due to its size, it may prey on small fish. Difficult during acclimatization from the wild. It requires plenty of oxygen, acidic, soft, or medium-hard water. Powerful filtration is necessary, including through peat, along with weekly water changes. Can be fed any aquarium foods, with a predominance of plant-based ones. pH 5.5–7.5; dH 3–20°; T 22–26°C.
Identification: Genus Hemiodus
Genus Hepsetus, Genus Erythrinus, Genus Hoplias, Genus Hoplerythrinus
Gasteropelecus levis, maculates, sternicla 
