The fire eel, which belongs to the spiny eels family Mastacembelidae, is often mistaken for a true eel due to its extremely elongated body, pointed snout, and underslung mouth. Its body is laterally compressed, especially at the rear third, where it flattens and joins the caudal fin to form an extended tail. The fire eel's base color is dark brown/grey, while its belly is lighter. The body is marked with several bright red lateral stripes and spots that vary in intensity depending on the fish's age and condition. In juvenile fish, the markings are usually yellow/amber, changing to a deep red in larger ones. The anal, pectoral, and dorsal fins often have a red edging.

Despite its name, the fire eel is not related to true eels but is instead the largest species in its family, capable of growing up to 1 m (3.3 ft) in length. Its natural coloration and elongated body make it an eye-catching addition to a large aquarium.