Spiny Eel

The Spiny Eel (Mastacembelus armatus), commonly known as the Tire Track Eel due to the tread-like zigzag pattern along its flanks, is one of the most recognizable large freshwater fish in the hobby. Belonging to the true spiny eel family Mastacembelidae rather than being a true eel, it is native to rivers, streams, and floodplains across South and Southeast Asia including India, Thailand, Vietnam, and the Malay Peninsula.

In the wild it can reach 90 cm in length, though aquarium specimens typically reach 50–70 cm. The body is muscular and elongated with a pointed snout bearing a fleshy, mobile rostral tip used to probe substrate and crevices for prey. A series of small spines runs along the dorsal surface ahead of the main dorsal fin, giving the family its common name. The intricate brown-and-cream zigzag pattern provides effective camouflage against sandy or gravelly stream beds.

The Spiny Eel is a nocturnal predator and bottom-dweller that spends daylight hours buried under sand or concealed beneath wood and rocks. At night it emerges to hunt worms, crustaceans, and small fish. Its burrowing ability is extraordinary — it can disappear completely beneath a sandy substrate — making substrate choice and tank security critical.

With a potential lifespan of around 10 years and impressive final size, this species demands substantial tank space and an experienced keeper. It is semi-aggressive toward small tank mates it can swallow and toward conspecifics in cramped conditions, but is otherwise a manageable and deeply fascinating long-term resident.

Tank Setup A minimum of 300 liters with a long footprint is required to house an adult Spiny Eel. Deep, fine sandy substrate of at least 10 cm is essential — this species burrows and must be able to submerge completely. Provide large caves, flat rocks, and driftwood for daytime hiding. A very secure, gap-free lid is mandatory; this species can push through surprisingly small openings and is a notorious escape artist.

Water Parameters Maintain pH between 6.0 and 7.5 with temperatures of 22°C to 28°C. Soft to moderately hard water is appropriate. Excellent filtration is needed to manage the waste produced by this large fish. Perform 25–30% water changes weekly.

Diet A carnivore that hunts primarily at night. Feed earthworms, prawns, mussel, and bloodworms — ideally at dusk or after lights-out. Live earthworms are eagerly taken and are an excellent staple. Sinking carnivore pellets may be accepted by trained individuals. Remove uneaten food the following morning.

Compatibility Semi-aggressive. Will consume small fish and shrimp. Best kept with robust, medium to large fish. Avoid small, slow, or bottom-dwelling fish that may be targeted. Multiple Spiny Eels can coexist in very large tanks but may compete for burrow sites.

Burrowing Behavior Expect the fish to be invisible for days at a time. This is normal. Do not disturb the substrate to check on the fish unless absolutely necessary. Provide enough substrate depth for complete burial.

Health Notes Sensitive to standard medications — use half doses and avoid copper-based treatments. Prone to skin lesions if kept on rough substrate. Ensure wound-free conditions by using fine sand exclusively.