Family Characidae or American Tetras ( Characidae )
Group of uncertain status ( Incertae Sedis )
Genus Thayeria.
This genus includes fishes known in English-speaking countries as penguin tetras due to their contrasting black-and-white coloration. Peaceful, schooling fishes, they get along well with other small species. In the wild, they swim in the upper water layer among dense vegetation and feed on insects falling onto the surface. They should be kept in aquariums planted with fine-leaved plants, with open space for swimming. Overly soft water is not recommended (10-20°dH); regular partial water changes and a temperature of 24-26°C are beneficial.
When swimming, the posterior part of their body is slightly lowered, and this oblique manner of movement is visually accentuated and highlighted by a longitudinal black stripe extending from the upper body onto the lower lobe of the caudal fin. They live for 3-4 years. Also kept in aquariums: — Thayeria sanctamariae (Ladiges, 1949) — Santa Maria penguin tetra. 7 cm. The black stripe starts right behind the dorsal fin and extends onto the lower lobe of the caudal fin.
— Boehlke’s penguin tetra, hockey stick tetra — Thayeria boehlkei [Weitzman, 1957]. (Blackline Penguinfish, Boehlke’s Penguin Fish, Hockey Stick Tetra). Inhabit tributaries of the Amazon River within Brazil and Peru. 5-6 cm. Silvery-green with a silvery sheen, the back is darker than the abdomen, the belly is silvery-white, and the fins are transparent. A velvety-black stripe about 3-4 mm wide runs from the gill cover to the tip of the lower lobe of the caudal fin, edged with two golden lines and resembling a hockey stick in shape. Unpaired fins have a bluish rim, and the adipose fin is white. Sexual dimorphism is poorly expressed. Males are smaller and slimmer than females, with an elongated lower lobe of the caudal fin.
Peaceful, best kept in a school in a community aquarium; suitable tankmates include small peaceful characins, rasboras, danios, most catfishes, and livebearers. The aquarium should feature dense plant thickets and floating plants, yet retain ample open space for swimming. They swim in the upper and middle water layers at an upward oblique angle, making rocking, seesaw-like movements. In nature, this swimming style helps the fish catch insects and their larvae falling onto the water surface.
Hovering motionlessly head-up, the fish seem poised to leap, but at the slightest sign of danger, they swiftly and collectively dart into plant thickets, resuming their previous positions once calmed down. Solitary specimens become timid and hide; thus, they should only be kept in a sizable school. Because penguin tetras are very skittish and can jump out of the tank when startled, the aquarium must be covered. It should be densely planted along the perimeter, leaving plenty of open space in the center for free swimming. Dark fine gravel is preferable as a substrate, as the fish will display their best colors against a dark background.
It is desirable to create moderate water flow in the aquarium. The water must be clean, as they are sensitive to nitrates; frequent water changes are necessary. Overly soft water is not recommended. Any live foods are suitable: bloodworms, tubifex, glassworms, small crustaceans, and dry foods can also be offered. They prefer feeding from the surface. They reach sexual maturity at 8-9 months of age. By 2 months, fry acquire adult coloration. Lifespan is 3-4 years. pH 6.0-7.0; dH 10-20°; T 22-27°C.
Identification: Blackline penguinfish — Thayeria boehlkei.
— Thayeria ifati [Gery, 1959]. Guiana, basins of the Maroni and Approuague rivers. 5 cm. Beige-gray, with a silvery belly and pinkish scales around the anal fin. A black stripe with golden edging begins in front of the dorsal fin, runs along the vertebral line, and extends onto the lower lobe of the caudal fin. Females are typically slightly larger and plumper when gravid.
These schooling fishes appreciate densely planted aquariums that still offer sufficient swimming space. Keep in a group of 6-10 specimens with an equal sex ratio. Overly soft water is not recommended (10-20°dH); regular partial water changes are beneficial. Omnivorous and peaceful community fish. Rarely encountered in the aquarium hobby. pH 6.0-7.2; T 23-28°C.
— Penguin tetra — Thayeria obliqua [Eigenmann, 1908]. Synonym: Thayeria sanctaemariae. (Penguin Tetra). South America, Brazil, Amazon, Rio Guaporé, and Rio Tocantins basins. 8 cm. The overall body coloration is silvery-green, with the back darker than the belly; fins are transparent and colorless. A broken black stripe, bordered with narrow gold stripes, starts below the dorsal fin, obliquely crosses the body towards the base of the caudal fin, and runs down its lower lobe. Fins are colorless. Sex determination is difficult. Males are smaller and slimmer. Females carrying eggs become visibly rounder and plumper in the abdominal area. In true *Thayeria obliqua*, the black stripe starts level with the dorsal fin, whereas in *Thayeria boehlkei* it begins at the operculum (gill cover).
A very calm, peaceful schooling fish characterized by an oblique swimming posture at a 45° angle, tail downward. Keep only with equally peaceful species. Lives in large schools. Inhabits the middle and upper water layers, where it requires plenty of swimming space alongside shelter in planted areas (including floating plants). Can tolerate a wide range of water parameters, including pH, temperature, and hardness. Overly soft water is not recommended; moderately hard water (10-20° dH) with a neutral pH is preferred, with regular partial water changes. Sudden shifts in water chemistry can be fatal. They relish live and frozen foods such as daphnia and bloodworms. pH 6-7; dH 10-20°; T 22-28°C.
Rachoviscus crassiceps, Xenagoniates bondi.
Paracheirodon innesi, Neon tetra, “Diamant”, “Gold”. 
