Sewellia sp. 'SEW02'

Overview

The provisional SEW02 designation refers to a hillstream loach whose identity remains unresolved. Its needs center on mature grazing surfaces and oxygen-rich moving water. A newly established aquarium is unsuitable, and the fish should not be bought simply to remove troublesome algae.

Distribution & habitat

Trade specimens are thought to come from the Hue area of central Vietnam, although their collection locality has not been verified. Related loaches occupy shallow headwaters and tributaries with rapid runs, rock shelves, gravel, and boulders. Clear sunlit water supports biofilm even where submerged plants are scarce.

Aquarium setup

Provide strong filtration and circulation, adding aeration where needed. A turnover around 15–20 tank volumes per hour is a useful target, alongside regular partial changes. Bright lighting supports algae; leave most surfaces available for grazing instead of scrubbing the entire aquarium. Exposed filter sponge offers another feeding area.

Seasoned wood and hardy attached plants can provide cover. Fresh wood that darkens the water undermines the lighting needed for algae growth. Stock only after the system has matured, and fit the lid closely because this loach can climb glass.

Diet & feeding

Algae and the microorganisms living among them form the main natural food; opportunistic insect feeding is only probable. Soft green growth and diatoms are preferable to tough brush algae. Vegetable-rich gel foods with spirulina can supplement grazing. When demand exceeds growth, rotate stones through a separate brightly lit algae-growing tank.

Check new arrivals for wasting. Severely depleted loaches may be difficult to rehabilitate and need continuous access to suitable food without competitors.

Behaviour & compatibility

Genus-level behavior suggests keeping at least six to spread disputes and reduce nervousness. Valuable feeding patches can be defended, often by dominant males, with rivals lifting themselves into upright, belly-facing contests.

Small current-tolerant open-water cyprinids and suitable stream gobies or catfish are potential companions. Shyer sucker-bodied loaches can lose access to food and space. Mixed bottom-dweller communities need generous room, broken sightlines, and abundant grazing surfaces.

Sexual dimorphism

Viewed from above, females have a fuller outline with a smooth transition from snout to pectoral fins. Males look narrower and more square-snouted, with the fins projecting sharply sideways. Mature males also develop rows of soft raised structures on the head and leading pectoral rays.

Breeding

Breeding attempts can borrow methods successful with other members of the genus; those successes do not establish a SEW02 breeding record. Mature rounded gravel shelters eggs and young, with algae-coated stones for adults and air-driven sponge filtration. A cooler water change is a possible stimulus.

In related loaches, pairs rise together and release gametes into the current. Their young initially forage within substrate crevices. Offer infusoria first, then Artemia nauplii and microworms once they forage above the gravel. Check external filters carefully for trapped young.

Notes

A visible gap separates the pelvic and anal fins, but the complete character combination does not fit earlier identifications. “Butterfly loach” is a broad trade label, so retain the SEW02 designation when discussing this form.