Feldmann's Tongue Sole

Geographical Origin and Habitat: Cynoglossus feldmanni is a freshwater flatfish native to Southeast Asia. It is primarily distributed in the Mekong and Chao Phraya river basins, spanning across Thailand, Cambodia, Laos, and parts of Indonesia (specifically Sumatra and Borneo) and Malaysia. It inhabits slow-moving rivers, canals, estuaries, and freshwater lagoons with very soft sandy or muddy bottoms, where it can easily bury itself to avoid predators and ambush prey.

Taxonomy and Morphology: Belonging to the family Cynoglossidae (tongue soles) and the genus Cynoglossus, this species exhibits a highly asymmetrical and laterally compressed body. Both eyes are located on the left side of the head, and the mouth is curved and situated ventrally. Pectoral fins are absent, and the dorsal, anal, and caudal fins are fused into a continuous fin fringe around the body. Adults grow to about 15-20 centimeters.

Social Behavior: It is a solitary, benthic, and mostly nocturnal species. During the day, it spends almost all of its time buried in the sand with only its eyes protruding to scan for threats and food. It has a peaceful, reclusive temperament and ignores other tankmates entirely, provided they are not small enough to be viewed as food, showing no territorial aggression towards other species.

Coloration and Sexual Dimorphism: The eyed (upper) side of the body displays a highly effective camouflage pattern of sandy browns, greys, and irregular dark mottling that mimics the riverbed. The blind (under) side, which rests against the substrate, is plain whitish or light pink. There is no visible sexual dimorphism, and males and females cannot be reliably distinguished based on external characteristics.

Aquarium Setup: Requires a specialized or quiet community aquarium of at least 150 liters. The single most important requirement is a deep bed (at least 5-7 cm) of very fine, smooth sand (such as silica sand). Coarse gravel or sharp substrates will cause physical abrasions to the fish's belly and eyes, quickly leading to fatal bacterial infections. The bottom area should be kept mostly open and free of rocks or heavy decor.

Diet and Feeding: This species is a strict carnivore and nocturnal predator. In captivity, it can be extremely difficult to feed, as it usually refuses dry flakes or pellets. It requires live or frozen foods that sink to the bottom, such as bloodworms, tubifex, small earthworms, brine shrimp, and blackworms. Feeding should occur after the aquarium lights are turned off, placing food close to where the sole is buried.

Water Quality: It demands clean, well-oxygenated water with stable parameters: temperature between 23°C and 28°C, pH range of 6.5 to 7.5, and water hardness (GH) of 5 to 15 dGH. While they can tolerate slightly brackish conditions, they thrive in pure freshwater. Regular water changes and efficient filtration are mandatory, but water flow should be kept low to moderate to prevent disturbing the sand bed.

Compatibility and Cohabitation: An entirely peaceful fish that is compatible with calm tankmates that occupy the middle and upper water columns, such as medium-sized tetras, rasboras, gouramis, or peaceful dwarf cichlids. Avoid housing them with aggressive or highly active bottom-dwellers (like loaches or territorial catfish) that might harass them, and do not keep them with very small fish or shrimp that could be eaten.

Aquarium Breeding: Breeding in home aquariums has not been documented. In the wild, they release pelagic eggs that drift in the water column. The larvae undergo a complex metamorphosis during their early development, during which the right eye migrates to the left side of the head, and the body flattens out before they settle onto the riverbed as juvenile flatfish.

Risks and Diseases: The primary risk is starvation due to its nocturnal feeding habits and reluctance to accept prepared foods, especially if kept with competitive tankmates. Bacterial skin infections are another common threat if the substrate is abrasive or dirty. Careful monitoring of food intake during night hours and a dedicated quarantine period are highly recommended.