Blue Star Damselfish

Blue Star Damselfish

The Blue Star Damselfish (Pomacentrus alleni), also called Allen's damselfish, is a small but intensely coloured reef fish — a glowing neon-blue body offset by a yellow belly and lower tail, with a fine black line tracing the lower edge. Hardy, inexpensive and brilliantly coloured, it brings an electric pop to a marine aquarium and is an easy fish for newcomers to the hobby.

As with most damselfish, that toughness comes with attitude: although tiny, it is bold and territorial, so its temperament should be factored into the stocking plan.

Natural Habitat & Origin

Pomacentrus alleni occurs in the eastern Indian Ocean, where it lives on coral reefs, staying close to the structure of branching coral and rubble into which it retreats when threatened. It establishes and defends a small territory around its chosen patch of reef.

In the aquarium it appreciates plenty of live rock with crevices and branching structure to claim as territory and shelter.

Care Requirements

Maintain stable marine conditions: salinity around 1.024–1.026, pH 8.1–8.4, and a temperature of about 24–26°C (75–79°F). Reaching only about 7 cm (3 inches), it can be kept in tanks of around 75 litres (20 US gallons) or more with ample rockwork. It is exceptionally hardy and tolerant, making it forgiving of beginner mistakes provided water quality is stable.

Diet & Feeding

The Blue Star Damselfish is an omnivore, feeding on zooplankton, algae and small invertebrates in the wild. It readily accepts aquarium foods: offer frozen mysis and brine shrimp, quality marine flakes and small pellets, and foods with marine algae content. Feed small amounts a couple of times a day; it is an undemanding, eager feeder.

Behavior & Temperament

This is a semi-aggressive, territorial fish that, like many damsels, punches well above its size. It can harass smaller or more timid tankmates, especially in a small tank, and is best added last so it cannot dominate the whole aquarium. It is bold and active, defending its patch of rock with surprising determination.

Tank Mates

Pair it with robust tankmates that can stand up to its attitude — clownfish, other damsels, larger wrasses and similar — rather than very shy or tiny fish. In a small reef, keeping a single specimen avoids the squabbling that occurs between multiple damsels. It is fully reef-safe, leaving corals and ornamental invertebrates alone.

Breeding

Like other damselfish, Pomacentrus alleni lays demersal eggs on a cleaned surface, guarded by the male, but rearing the pelagic larvae is rarely accomplished in the home aquarium. Trade specimens are wild-collected.

Common Health Issues

The Blue Star Damselfish is among the hardiest marine fish, but it can still contract marine ich (Cryptocaryon irritans) or marine velvet (Amyloodinium ocellatum) under stress. Quarantine new arrivals and keep water quality stable. Mindful of its bold temperament when choosing tankmates, it is otherwise an easy, colourful and resilient fish — and a good choice for a first marine aquarium or a splash of blue in a reef.