Gudgeon (fish)
Gudgeon is the common name for a number of small freshwater fish of the families Butidae, Cyprinidae, Eleotridae or Ptereleotridae. Most gudgeons are elongate, bottom-dwelling fish, many of which live in rapids and other fast moving water.

Eleotridae – Known commonly as gudgeons, many species in the family Eleotridae are also called sleeper gobies. Unlike gobies, Eleotridae gudgeons have paired ventral fins rather than a fused ventral fin. In Australia, gudgeons from the family Eleotridae are widespread and are popular for aquariums.
Ptereleotridae – Gudgeons in the family Ptereleotridae are primarily marine species and are often associated with tropical coral reefs.