Two Easily Confused Species
The "Penguin Tetra" is actually two related species, "Thayeria boehlkei" and "Thayeria obliqua". The easily seen differences between these species are quite small. Both species have a tendency to swim in a tail down position, but this is more pronounced in Thayeria obliqua. Thayeria obliqua has a shorter black line than Thayeria boehlkei, and Thayeria obliqua grows a little bigger but these are both small fish. The species most often available in aquarium shops is Thayeria boehlkei. Some people prefer to keep the name "Penguin Tetra" for Thayeria obliqua and call Thayeria boehlkei the "False Penguin Tetra". There are several other common names for this fish, some of them, like "Penguinfish", Penguin Fish", etc are also based on its perceived resemblance to a penguin, while others like "Short Striped Thayeria" do not. In Australia, it is often sold as a "Hockey Stick Tetra" and the shape of the prominent black line is reminiscent of the shape of a hockey stick.
Thayeria boehlkei grows to about one and a half inches (4cm) long while Thayeria obliqua can get a little bit bigger. They come from the Amazon River and its tributaries in Brazil and Peru, preferring slow moving stretches with a lot of plants.
The Penguin Tetra is a tropical fish. 24̊ C (75̊ F) is a suitable temperature to set your thermostat. The water they are used to in the wild is soft and acid, but in captivity they will adjust to neutral pH and some hardness.
The Penguin Tetra does better in clean water and care needs to be taken with the water quality. They are sensitive to Ammonia, Nitrite and excessive concentrations of Nitrate.
Like many tetras, the Penguin Tetra is an easily fed omnivore. It will eat all normal aquarium foods, but will enjoy some live food like mosquito larvae and Daphnia. Frozen foods, including frozen blood worms are also appreciated.
The Penguin Tetra is a peaceful schooling fish. I suggest a minimum of four, and preferable more be kept together. They can be kept with most small, reasonable peaceful fish, including: Red Eye Tetras, Cardinal Tetras, Silvertip Tetras, Gold Barbs, Cherry Barbs, Pristella Tetras, Rummy Nose Tetras, White Cloud Mountain Minnows, Black Widow Tetras, Rosy Barbs, and Colombian Tetras, as well as Peppered Catfish.
A school of Penguin Tetras can usually even be kept with slow long finned fish like Siamese Fighting Fish, Guppies and Endlers Guppies, although I would not be sure in every case.
There is not much difference between the sexes of the Penguin Tetra, but when in breeding condition, the females get a rounder belly.
Thayeria boehlkei is the Penguin Tetra most likely to be available in shops and this fish is reasonably easy to breed. I suggest a water level lowered to about 6 inches (15cm) acidic and very soft water with a temperature of about 27̊ C (81̊ F). There should be fine leaved, bushy plants in the breeding tank.
The Penguin Tetra is a prolific breeder in the sense of producing a lot of eggs. A female of this little fish with a very well rounded belly can produce as many as 3000 eggs. These are very small. The male produces a large amount of sperm to try to fertilize all these eggs, and this can contribute to fouling the water. Partial water changes are recommended to maintain the high quality of water that the fry need. The addition of an aquarium fungicide is recommended to help protect the eggs from fungus early on.
The eggs should hatch in as little as 12-18 hours. The babies are very small, and need protozoa (infusoria) at first.
Although the Penguin Tetra is a delight in an aquarium, as with all pets, do not allow them to get into ecosystems they are not native to.
Aquarist are frequently blamed for this, and there is no doubt that there are cases of accidental, or deliberate, release of fish by aquarists, but my own research suggests that the worst culprits for this historically have been governments and their departments.
Fishermen are also not totally blameless; see Lake Eacham Rainbowfish Fact Sheet to see how a species became "extinct" through the introduction of fish from other ecosystem.
I am grateful to the following sources of information: Data Base, Aqua World, Fish Profiles.com, Munga Bay, Aquatic Community, Badman's Tropical Fish and Live aquaria.com.