tetra
n. countablen. a small, colorful tropical fish that people often keep in home aquariums. They usually live in groups and come from rivers in South America and Africa.
n. any of various small, often brightly colored freshwater fish of the family Characidae, popular in the aquarium trade. Typically refers to species within the genera Paracheirodon or Hyphessobrycon.
I bought five neon tetras for my new fish tank.
The school of tetras darted through the aquatic plants, their silver scales catching the light from the aquarium lamp.
While the neon tetra is perhaps the most iconic, many hobbyists prefer the cardinal tetra for its more extensive red coloration and hardiness in established blackwater setups.
The plural can be either 'tetra' or 'tetras', though 'tetras' is more common when referring to multiple individuals.