Threadfin Goby (Sicyopterus Longifilis)

Dorsal rays trail like threads through the current.

The Threadfin Goby (Sicyopterus longifilis) is a striking, algae-grazing freshwater goby that thrives in fast-flowing, highly oxygenated rocky streams.

• Water conditions: The Threadfin Goby is a rheophilic species built for swift, cool, oxygen-rich water. Aim for a temperature of 22–26°C (71–79°F), pH 6.8–8.0, and general hardness around 5–15 dGH. Ammonia and nitrite must remain at zero, with nitrate kept low, ideally below 20 ppm. Provide strong, continuous current with a turnover of roughly 10–20 times the tank volume per hour using canister filters, powerheads, and spray bars, while creating quieter eddies and resting spots among rocks. High dissolved oxygen is essential, so position returns to agitate the surface. Stability matters more than exact numbers: avoid temperature swings, perform regular partial water changes with pre-matched water, and keep the tank mature. Adults are freshwater fish and do not require brackish water, though wild-caught specimens may need careful acclimation. This is not a fish for a newly cycled aquarium.
• Suitable tank mates: Choose peaceful, current-loving species that tolerate strong flow and cooler conditions. Good companions include hillstream loaches such as Gastromyzon and Sewellia, Garra species, danios such as Devario and Danio, slender rasboras, and other peaceful rheophilic gobies. Small, robust loaches and fast-water catfish may also work in a sufficiently large, well-structured setup. Avoid slow-moving fish, long-finned species, bettas, fancy guppies, delicate shrimp, and aggressive or predatory cichlids. Threadfin Gobies are generally peaceful but males can spar over territory, so provide plenty of smooth rocks, caves, and visual barriers. A tank of at least 90 cm (36 inches) is advisable to create a meaningful current gradient and enough grazing surface. Do not combine them with fish that compete heavily for biofilm unless the tank is large and mature enough to support both.
• Feeding habits: In the wild, Sicyopterus longifilis is an algivore and biofilm grazer, using specialised teeth to scrape algae and diatoms from rocks. In captivity, a mature tank with algae-covered stones is extremely valuable, but it should not be the only food source. Offer spirulina and kelp-based wafers, Repashy Soilent Green or similar gel foods, nori, blanched zucchini, cucumber, spinach, and other vegetable matter. Some specimens accept small amounts of frozen foods such as baby brine shrimp or daphnia, but these should be treats rather than staples. Feed small portions several times daily and remove uneaten food to protect water quality. Because they graze almost constantly, a clean, sterile aquarium often leads to starvation. Monitor the belly and overall body condition; a pinched look is a warning sign. Supplemental algae paste smeared on rocks can help newly acclimated fish.
• Care requirements: This is an advanced fish best suited to experienced aquarists. Acclimatise new arrivals slowly with drip acclimation, quarantine where possible, and avoid exposing them to abrupt changes in temperature or chemistry. The aquarium must be tightly covered, including small gaps, because Threadfin Gobies can climb glass and jump. Build a rocky layout with smooth boulders, cobbles, caves, and perching ledges, leaving open swimming lanes in the current. Keep the water surface moving strongly and maintain excellent oxygenation. Avoid overstocking, heavy feeding, and fluctuating parameters. These gobies are sensitive to poor water quality and may decline quickly if conditions slip. Breeding is not realistic in the home aquarium because the larvae are amphidromous and require a brackish or marine phase. Handle fish as little as possible, keep them in a stable, mature system, and observe them regularly. With the right fast-water setup, abundant grazing surfaces, and consistent care, the Threadfin Goby is a fascinating and active specialist for the rheophilic community tank.