Plectranthias inermis is a fish with an identity crisis. It is commonly sold in the aquarium trade as the geometric hawkfish, but it is usually called the unarmed perchlet. It looks like a hawkfish (at least superficially) and acts like a hawkfish, but it is not closely related to the hawkfish (family Cirrhitidae). It is a perchlet. Perchlets are members of the family Serranidae (the groupers) and the subfamily Anthinnae (the anthias or fairy basslets). If you know anything about anthias, it will probably surprise you that perchlets are related. They certainly look more like a hawkfish! Of the 40-odd species in the genus, only the geometric hawkfish (Plectranthias inermis) is seen with any regularity in fish stores. The geometric hawkfish has become a rather popular species with reefkeepers.
Difficulty: The geometric hawkfish is good for beginners to advanced hobbyists and will thrive in a nano-reef to the largest reef aquarium system. The geometric hawkfish is a durable tropical fish that accepts a wide range of foods, including finely chopped seafood (e.g., shrimp, squid, scallop), frozen fish eggs, frozen foods for carnivores and mysid shrimp. Some geometric hawkfish will also consume flake food once it sinks toward the bottom.
Physical Description: Not only is the geometric hawkfish shaped like a hawkfish, it also shares common chromatic attire to some cirrhitids (e.g., Falco’s hawkfish, Cirrhitichthys falco). The geometric hawkfish is white overall, with red blotches and mottling. The geometric hawkfish reaches a length of about 2 inches, with the male getting slightly larger (around 0.5 inches longer) than the female. The male geometric hawkfish also develops a larger pectoral fin as it reaches its maximum length.
Range: The geometric hawkfish is known to occur on reefs off of Christmas Island (off the western coast of Australia), Indonesia, Papua New Guinea and the Philippines. The geometric hawkfish is a resident of fore reef drop-offs and slopes and has been recorded at depths of 45 to over 200 feet. It is usually found among rubble or resting in reef crevices.
Compatibility: Although the geometric hawkfish is a predator that feeds on small crustaceans and newly settled fish, its small size means that it is not a great threat to invertebrate and fish tankmates. In fact, the geometric hawkfish is more likely to be eaten by other piscivores, such as morays, lizardfish, squirrelfish, frogfish, scorpionfish, groupers (even smaller sea basses), snappers and hawkfish. The geometric hawkfish is usually indifferent toward non-related species. You can keep more than one Plectranthias inermis in the same aquarium, but they should be introduced together, and it is best if you add one male Plectranthias inermis per aquarium (with either another female or two or three females depending on the aquarium size). Because the geometric hawkfish is most likely a protogynous hermaphrodite (males result from female sex change), if you add two smaller individuals, the more dominant one may change into a male, while the other may develop functional ovaries.
Aquarium Conditions: The geometric hawkfish is somewhat reclusive, peering out of interstices and perching near the entrance to a shelter most of the time. In the aquarium, provide the geometric hawkfish with lots of available refuge sites that it can retire to when it feels uneasy. It is possible that the geometric hawkfish may leap from an aquarium, especially if it is being threatened by more pugnacious neighbors.
Care Considerations: Plectranthias inermis is a wonderful little fish for the nano-reef aquarium. Because of its diminutive size and somewhat retiring nature, the geometric hawkfish is likely to get lost in a more expansive reef aquarium.
Breeding: While there are no reports of the geometric hawkfish spawning in the home aquarium, this tropical fish is an ideal candidate to do so. The geometric hawkfish is small enough that in a larger aquarium, it should be able to complete a spawning ascent (where the fish dart into the water column and release their gametes at the top of the ascent before darting back to the safety of the rockwork).