Black Axil Chromis Chromis Atripectoralis

Black Axil Chromis (Chromis atripectoralis)

Overview

The Black Axil Chromis is a peaceful, schooling damselfish often mistaken for the Blue-Green Chromis (Chromis viridis). The key difference is the distinct black spot at the base of the pectoral fin (“axil”), which gives the species its name. Adults reach around 10 cm (4 inches) and display colours ranging from shimmering green to blue, depending on lighting. Their hardy nature, active swimming, and peaceful temperament make them ideal community reef fish.

Origin

Widespread across the Indo-Pacific, including the Red Sea, Great Barrier Reef, Fiji, Indonesia, and Micronesia. They are commonly seen in large shoals above branching corals on lagoon and seaward reef slopes.

Feeding

The Black Axil Chromis is a planktivore that readily adapts to aquarium diets:

• Marine flakes and high-quality micro-pellets (staple)

• Frozen mysis, cyclops, and enriched brine shrimp

• Occasional finely chopped seafood

• Spirulina or algae-based foods for colour and health

Best kept on 2–3 small feedings per day to mimic their natural grazing behaviour.

Tank Mates

Peaceful and adaptable, they mix well with most reef-safe fish, such as:

• Clownfish

• Gobies and blennies

• Fairy and flasher Wrasses

• Cardinalfish

• Other chromis species (with space)

Avoid large aggressive predators, which may target them.

Aggression

Peaceful to mildly territorial. While generally docile, they may show dominance behaviour within their shoal. Keeping them in groups of 5+ spreads aggression and encourages natural schooling.

Experience Level

Beginner-friendly. Hardy, forgiving of minor water quality fluctuations, and easy to feed, making them an excellent starter marine fish.

Water Parameters

• Temperature: 24–27°C (75–81°F)

• pH: 8.1–8.4

• Salinity: 1.020–1.025 SG

• Hardness (dKH): 8–12

• Tank Size: Minimum 30 gallons (115L) for a small group; larger systems allow for bigger shoals and more natural behaviour

Provide plenty of open water for active swimming along with coral branches or rockwork for shelter.

Reef Compatibility

Reef safe. They will not harm corals or invertebrates and are a classic choice for reef aquariums.