Masked Swallowtail Angelfish

Masked Swallowtail Angelfish

The Masked Swallowtail Angelfish (Genicanthus semifasciatus) is one of the prized reef-safe angelfish — a graceful, mid-water plankton feeder rather than a coral-grazer. Males and females are strikingly different: females wear a dark "mask" over the head and dark upper body, while males develop golden-yellow markings and fine striping. As a member of the swallowtail genus Genicanthus, it is peaceful, elegant and one of the few angelfish that can be trusted in a full reef.

A deeper-water, slightly cooler-living species, it brings refined movement to the open water of a marine aquarium.

Natural Habitat & Origin

Genicanthus semifasciatus is found in the western Pacific, around southern Japan, Taiwan and the Philippines, where it lives on deeper, current-swept reefs, forming midwater schools and harems of a male with several females. It feeds on plankton drifting past in the current.

In the aquarium it appreciates open swimming space, steady flow, and rockwork for shelter, plus slightly cooler temperatures than typical shallow-reef fish.

Care Requirements

Maintain stable marine conditions: salinity around 1.024–1.026, pH 8.1–8.4, and a temperature toward the cooler end of the range, around 23–26°C (73–79°F). Reaching about 21 cm (8 inches), it needs a roomy tank of around 340 litres (about 90 US gallons) or more with open water to swim. Subdued lighting during acclimation helps a deeper-water fish settle; healthy, well-acclimated specimens are hardy.

Diet & Feeding

This swallowtail is essentially planktivorous, feeding on zooplankton in the wild along with some benthic invertebrates and algae. It accepts aquarium foods well: offer frequent small meals of frozen mysis and enriched brine shrimp, marine angel preparations and foods with marine algae content, suiting its natural midwater grazing.

Behavior & Temperament

This is a peaceful, sociable angelfish that can be kept singly, as a pair, or as a male with several females in a large tank. It is not the territorial bully many angels are, and mixes well in a calm community. It spends its time cruising open water rather than picking at the rocks.

Tank Mates

Good companions are other peaceful reef fish — tangs, anthias, wrasses, gobies and similar — and it generally coexists well even with other peaceful angels given space. Crucially, G. semifasciatus is reef-safe: as a planktivore it leaves corals and ornamental invertebrates alone, making it one of the few angelfish suitable for a full reef.

Breeding

Genicanthus semifasciatus is a protogynous hermaphrodite that forms harems, with a dominant female changing sex to male. It is a pelagic spawner, and rearing the larvae is beyond the home aquarium, so trade specimens are wild-collected from deeper reefs.

Common Health Issues

Given good acclimation, this angelfish is reasonably hardy, but like all marine fish it is susceptible to marine ich (Cryptocaryon irritans) and marine velvet (Amyloodinium ocellatum), particularly when stressed by deep-water collection and transport. Quarantine new arrivals, acclimate gently with subdued lighting, and keep water quality stable. A well-chosen specimen rewards the effort as a peaceful, reef-safe angelfish of real elegance.