Budding aquarists often begin their tanks with some type of tetra (Characins). Tetras are very hardy fish that get along with several different types of community fish, although some can be a bit fin nippy (Serpae Tetras for example). These fish have a tendency to become stressed when paired with fast moving fishes or fish that dart around constantly. Tetras will get along with most community fish of a similar size. The one strict rule of freshwater aquariums is if a fish can fit in another fishes mouth, then eventually it will fit in that fishes mouth.
When selecting tank mates for small tetras like Neon and Glowlight tetras, it is best to keep the fish relatively the same size. There are several good tank mates of similar size, like White Clouds, Fancy Guppies, Corydoras Catfish, Glass Fish, Ghost Shrimp, Harlequin Rasboras, Hatchetfish, Pencilfish, Zebra Danios, X-Ray fish and Cherry Barbs. It is also fine to keep tetras with vegetarian fish, such as Banjo Catfish, Bristlenose Plecostomus, Otocinclus, Kuhli and Dojo Loaches.
Larger tetras can be kept with faster moving and larger tank mates. Black Skirt Tetras, Painted Tetras, Serpae Tetras, Red Eye Tetras, Black Neon Tetras and Silver Tip Tetras all do well with larger tank mates like Giant Danios, Swordtails, Mollies, Platies, Dwarf Gourami, Flame Gourami,Opaline Gourami, Pearl Gourami, Betta (as long as there is plenty of plantlife to share). Others good tank mates include the fancy Spiketailed Paradisefish, Green Catfish, Leopard Catfish, Common Plecostomus, Spotted Catfish, Upside Down Catfish, Angelfish (depending on the temperament of the Angelfish), the vivid Blue Acara, Festivum, Ram, juvenile Bala Shark, Black Shark, Rainbow Shark, Blue Danio, Clown Loach, Dwarf Loach and Horseface Loach.
In a tank filled with semi aggressive fishes like Honey Gourami, Kissing Gourami, Bumble Bee Catfish, Flying Fox, Green Tiger Barb, Red Tailed Shark, Rosy Barb, Tiger Barb, Tinfoil Barb, Firemouth or Severum, it is best to keep fast moving and strong willed tetras. These include the Bleeding Heart and Buenos Aires Tetras, the aptly named Black Widow (tetra) and the largest of the Characins, the Silver Dollar.
Tetras may be hardy fish, but there are some species that they should absolutely never be kept with. Most of these are highly aggressive fish that will make a meal out of even the most feisty of tetras. These fish include the Red Belly Piranha, Black Paradisefish, Convict, Green Terror, Jack Dempsey, Red Oscar, Black Ghost Knifefish and Butterfly Fish. Tetras should also not be kept with Goldfish. Goldfish are very dirty fish and the high levels of pollutants they produce can cause poisoning in the tetra community. Even though some parties claim Chinese Algae Eaters are fine tank mates, they tend to grow to a large size and then begin preying on the tetras when algae levels get too low. They will attach to the side of the fish and eat a hole near the stomach area and kill it.