Allen's Damselfish

Allen's damsel is a small Pomacentrus species that stands out with its bright metallic blue body from head to tail. Its anal fin is yellow, and a black line runs along the lower lobe of its tail. In species it is often confused with in stores, such as the neon damsel (Pomacentrus coelestis) and the goldbelly damsel (P. auriventris), the yellow also extends onto the belly or tail; in Allen's damsel the yellow is only on the anal fin.

It reaches about 6 cm. It is calmer than many species of the genus Pomacentrus, but its territoriality increases with age. It is hardy; still, it is not as tough as some damsels and does better in a mature reef aquarium than in a newly set-up one.

It lives in the Andaman Sea (Thailand's Similan Islands and surroundings) and Indonesia, on outer reef slopes and in nearshore areas at depths of 3–15 meters. It is seen in small groups along the edges where the reef meets the sand, above piles of rubble and dead coral. It feeds on plankton in the water column and on small organisms on the bottom.

• Volume: at least 110 liters for a single fish; 250 liters or more for a group.
• Water: salinity 1.023–1.026 specific gravity, pH 8.1–8.4, temperature 24–27°C; low nitrate.
• Cycling: it should be added to a stable aquarium that has finished cycling.
• Layout: plenty of rocks, caves and crevices; open swimming space at the front.
• Reef compatibility: reef safe; it does not touch corals or invertebrates.

Omnivorous. Make quality granules and flakes the staple; add frozen mysis, brine shrimp and copepods. Offer foods containing spirulina or algae several times a week. Feeding small amounts 2–3 times a day keeps the colors vivid.

Active and curious. It is territorial toward its own kind and similar blue damsels; it mostly lives without problems with peaceful fish of a different shape. It may be harsh toward small fish added to the tank later, during their first days.

Good companions: clownfish, gobies, blennies, cardinalfish, dwarf angelfish, tangs, cleaner shrimp.

Avoid: other blue damsels, aggressive damsel species, large dottybacks and predators that can fit it in their mouths.

In a medium-sized aquarium keep it singly. In a large aquarium a group of 5–6 juveniles can be added at the same time; because aggression is spread within the group, it is directed less at other fish. Do not add individuals of the same species one at a time and at different times.

It forms pairs in the breeding season. The female lays her sticky eggs on a rock surface cleaned by the male; the male guards the eggs and fans them with his fins. The eggs hatch within a few days. Raising the planktonic larvae requires a rotifer culture and a separate container; it is rare in home aquariums.

• Marine ich and velvet: quarantine new fish for 2–4 weeks.
• When frightened or stressed its color fades and turns brownish; a persistent fading is a sign of a problem.
• Poor water quality and inadequate feeding cause loss of color.

There is no reliable external difference. Males are often slightly brighter in color; in the breeding season the female's belly becomes fuller.