Blue Damselfish (Chrysiptera cyanea)

The Blue Damselfish, also known as the Blue Devil Damselfish, is one of the most iconic and strikingly vibrant fish in the marine hobby. It is famous for its electric, "glowing" sapphire-blue body. However, its beauty comes with a warning: it is widely considered one of the most aggressive and territorial small fish in the trade, earning its "Devil" moniker by fiercely defending its territory against fish many times its size.

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Key Information

• Common Names: Blue Damselfish, Blue Devil, Sapphire Damselfish, Sky-blue Damselfish

• Scientific Name: Chrysiptera cyanea

• Origin: Indo-Pacific (Western Australia to the Marshall Islands)

• Max Size: 8.5 cm (approx. 3.3 inches)

• Lifespan: 5–10+ years

• Temperament: Highly Aggressive and territorial

• Diet: Omnivore – Accepts almost any prepared aquarium foods

• Care Level: Very Easy (Nearly "indestructible")

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Sexual Dimorphism (Telling Males from Females)

Unlike many other damselfish, it is easy to tell the gender of C. cyanea:

• Males: Features a solid blue body with a bright orange-to-yellow tail. In some regions, males also develop an orange nose.

• Females: Usually entirely blue and lack the orange tail. They often have a small black spot at the base of the rear dorsal fin.

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Behaviour & Tank Compatibility

• The "Devil" Personality: This fish is a powerhouse of aggression. It will claim a specific rock or cave and relentlessly harass any fish that enters its "zone."

• Tank Mates: Should only be housed with robust, assertive fish such as Tangs, Large Angelfish, Dottybacks, or large Wrasses. Avoid keeping them with peaceful species like Firefish, Cardinals, or small Gobies, as the Blue Devil may kill them.

• Social Structure: Best kept singly unless you have a very large tank (500L+) with a massive amount of rockwork to break lines of sight.

• Reef Safe: 100% Reef Safe. They are great for coral health as they do not nip polyps and their activity can help keep detritus off rockwork.

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Ideal Tank Setup

• Tank Size: A minimum of 115L (30 Gallons) is recommended for a single specimen.

• Rockwork: Provide excessive amounts of live rock with many hiding places. This allows the Blue Devil to feel secure in its territory, which might slightly reduce its desire to hunt down other tank mates.

• Water Quality: They are legendary for their hardiness and can survive in conditions that would be fatal to other fish, making them a common (though controversial) choice for new tanks.

• Lid: Like all damselfish, they are fast and can jump if they get into a high-speed chase. A secure lid is needed.

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Why Choose the Blue Damselfish?

The Blue Damselfish is for the hobbyist who wants the most intense blue possible in an aquarium and isn't afraid of a fish with a "mean" streak. It is a perfect choice for a Fish-Only-With-Live-Rock (FOWLR) system with other aggressive inhabitants. If you want a fish that is active, never hides, and is practically bulletproof in terms of health, the Blue Devil is a striking, albeit bold, choice.