Gymnothorax polyuranodon

The Freshwater Moray Eel is a beautiful, large, and snake-like fish with a striking pattern of gold or leopard-like spots, which is why it's also called the Tiger or Goldspotted Moray. Despite its name, this is not a true freshwater fish; it must be kept in brackish water, which is a mix of fresh and saltwater, to stay healthy. Keeping one is a serious, long-term commitment, as they can live for over a decade and grow quite large, requiring a very spacious aquarium of at least 450 litres.

This eel is a predator that should be housed alone. It will eat any fish or shrimp small enough to fit in its mouth. They are not active swimmers, preferring to spend most of their time hiding in caves or pipes with just their head poking out, waiting for food. You'll only need to feed it meaty foods like shrimp or pieces of fish about once a week. Be very careful with your aquarium lid! These eels are notorious escape artists and can squeeze through the smallest gaps. Because of their specific water needs, large size, and predatory nature, they are considered difficult to care for and are only recommended for experienced fish keepers.

Gymnothorax polyuranodon, commonly but inaccurately known as the Freshwater Moray Eel, is a fascinating species for the specialist aquarist. Its 'hard' difficulty rating stems from several key requirements, chief among them being its need for a stable, low-flow brackish environment. While often sold as a freshwater species, it will fail to thrive and ultimately perish without the addition of marine salt to create brackish conditions, ideally with a specific gravity between 1.005 and 1.012. Maintaining stability in this environment is crucial, as are the warm, alkaline water parameters it originates from in the Indo-Pacific estuaries.

An appropriately sized aquarium is non-negotiable. Given its potential to reach nearly a meter in length, a tank with a large footprint is more important than height to accommodate its bottom-dwelling and sedentary lifestyle. The aquascape should be rich with secure hiding places. A network of PVC pipes with a diameter large enough for the eel to turn around in, combined with stable rockwork creating caves and crevices, is essential for its well-being. A lack of secure hiding spots will cause chronic stress. A heavy, tightly-sealed lid with no gaps is mandatory, as this anguilliform-bodied fish is a master of escape.

As a carnivore with a low metabolic rate, a weekly feeding schedule is sufficient. Target-feeding with tongs is the recommended method to ensure the eel eats and to prevent accidental bites. While they may initially require live food, they can be weaned onto frozen/thawed items like prawns, silversides, and clam meat. Despite infrequent feedings, they are high waste producers. A single large meal creates a significant bioload, necessitating a powerful and mature biological filtration system to process the ammonia spike. Regular, large-volume water changes are critical to manage nitrate levels and maintain water quality.

Socially, this is a solitary and territorial species. Housing one per tank is the safest approach. While they are slow swimmers, they are opportunistic ambush predators; any tank mate small enough to be ingested will be viewed as food. Even with larger, robust brackish companions, aggression can be an issue as the eel matures. Breeding is not a viable goal in the home aquarium, as it is believed to have a complex reproductive cycle that involves a marine larval stage, a feat that has not been replicated in captivity.

Gymnothorax polyuranodon is a member of the Muraenidae family, making it a true moray eel. The common name 'Freshwater Moray Eel' is a commercial misnomer that causes significant husbandry issues. This species is properly classified as euryhaline, demonstrating a wide tolerance for salinity fluctuations, but it is physiologically dependent on brackish conditions for long-term health. Its natural habitat includes the low-energy, mangrove-lined estuaries and lower river reaches of the Indo-Pacific region.

Its anguilliform body shape is a classic adaptation for a sedentary, benthic predator, allowing it to navigate complex rockwork and burrow systems with minimal energy expenditure. This morphology, combined with a confirmed low metabolic rate, supports its ambush predatory strategy and infrequent feeding requirements. From a husbandry perspective, its carnivorous diet leads to the excretion of high concentrations of nitrogenous waste (primarily ammonia) following each feeding event. This high bioload factor, disproportionate to its feeding frequency, demands an oversized biological filtration capacity to prevent acute water quality degradation. Aquarists must manage the system based on the post-feeding spike rather than the eel's low daily activity level.

Behaviorally, G. polyuranodon is highly territorial and asocial, consistent with many Muraenidae. In a captive environment, it establishes a territory centered around a preferred hiding structure. Its sensory system is more reliant on chemoreception (smell) than sight, which it uses to hunt, primarily at dawn and dusk. This reliance on smell can lead to it mistakenly striking at an aquarist's hand during feeding or maintenance.

Reproduction in captivity is undocumented and considered unattainable. It is presumed to follow a catadromous or oceanodromous life cycle, wherein adults spawn in or migrate to full marine environments. The eggs hatch into leptocephalus larvae, a distinct, transparent, ribbon-like planktonic larval stage common to all Elopomorpha. These larvae drift in pelagic marine environments for an extended period before metamorphosing into glass eels and recruiting into coastal brackish habitats. This complex life cycle presents an insurmountable barrier to captive breeding. According to the IUCN Red List, Gymnothorax polyuranodon is listed as 'Least Concern' (LC), indicating a stable and widespread wild population.