Squareback Anthias - Male

The male Squareback Anthias (Pseudanthias pleurotaenia), also known as the Squarespot Anthias, is a spectacular saltwater fish prized for its intense pink, magenta, and reddish coloration. Mature males develop a distinctive pale square or rectangular patch along the side of the body, along with an elongated dorsal spine, blue-to-violet fin accents, and a deeply forked tail. These dramatic features make the male one of the most visually striking anthias for a spacious marine aquarium.

Native to the Indo-Pacific, this species inhabits outer reef slopes, drop-offs, and current-swept reef environments where groups swim above the reef and feed on passing zooplankton. Its constant midwater activity and brilliant coloration make it especially appealing for large reef aquariums and appropriately sized fish-only systems.

Reaching approximately 8 inches as an adult, the Squareback Anthias is considerably larger than many other anthias species and requires substantial open swimming space. Males can also become territorial toward other anthias, particularly rival males, so long-term success depends on adequate aquarium size, secure retreats, and thoughtful social stocking.

Quick Care Guide

Care Level: Moderate
Minimum Tank Size: 125 gallons
Temperament: Semi-aggressive
Diet: Carnivore / Planktivore
Reef Safe: Yes
Region: Indo-Pacific
Scientific Name: Pseudanthias pleurotaenia

Tank Requirements

Provide a mature, well-established marine aquarium with extensive open swimming space above and around the rockwork. Male Squareback Anthias are large, active water-column swimmers and benefit from a long aquarium footprint with plenty of unobstructed room to move.

Arrange rockwork to create caves, ledges, overhangs, and shaded retreats while preserving substantial open water. These protected areas provide nighttime shelter and allow the fish to retreat from territorial interactions when necessary.

Moderate to strong, well-distributed water movement is ideal and helps replicate the current-rich reef habitats where this species naturally feeds. Strong oxygenation and efficient filtration are important because the frequent feeding required by anthias can increase nutrient input.

A securely fitted mesh screen or aquarium lid is recommended, as anthias may jump when startled or chased.

Diet and Feeding

The Squareback Anthias is a carnivorous planktivore that naturally feeds on zooplankton and small crustaceans suspended in the water column.

Offer a varied diet including enriched mysis shrimp, enriched brine shrimp, copepods, calanus, frozen plankton blends, finely chopped marine seafood, and appropriately sized high-quality marine pellets or flakes. Foods that remain suspended in the water column help encourage natural feeding behavior.

Feed two to three small meals per day, with additional small feedings beneficial when practical. Newly introduced specimens may initially respond best to mysis shrimp, copepods, and other frozen planktonic foods before readily accepting a broader prepared diet.

Temperament and Tankmates

Male Squareback Anthias are generally compatible with peaceful to moderately assertive marine fish but can become territorial toward other anthias. Mature males may display, chase subordinate individuals, and aggressively challenge competing males.

Suitable tankmates include tangs, rabbitfish, peaceful to moderately assertive angelfish, fairy and flasher wrasses, clownfish, gobies, and other non-predatory marine fish that will not continually intimidate or outcompete them.

Multiple mature males should generally be avoided unless the aquarium is exceptionally large and provides substantial separation between territories. A single male can be maintained successfully, or one male may be housed with several females in a suitably spacious aquarium.

Squareback Anthias are protogynous hermaphrodites. If the dominant male is removed from an established group, a dominant female may transition into a male and assume the dominant social role.

Reef Compatibility

The male Squareback Anthias is considered reef safe and is an excellent choice for large coral-dominated aquariums. It does not normally consume or damage stony corals, soft corals, clams, or other sessile reef organisms.

It is also generally compatible with ornamental shrimp, crabs, snails, and other established reef invertebrates. Tiny copepods and other planktonic crustaceans may naturally be consumed as part of its normal feeding behavior.

Water Parameters

Temperature: 75–80°F
Specific Gravity: 1.024–1.026
pH: 8.1–8.4
Alkalinity: 8–12 dKH

Maintain stable, highly oxygenated water with consistently good water quality. Efficient filtration and regular nutrient management are particularly important because this active species benefits from multiple daily feedings. Stability is more important than targeting one exact value within these ranges.

Common Considerations

• Large adult size: Can reach approximately 8 inches and requires substantial open swimming space.

• Male coloration: Mature males develop vivid pink-to-magenta coloration with a distinctive pale square or rectangular flank patch.

• Male territoriality: May chase subordinate anthias and aggressively challenge competing males.

• Frequent feeding: Benefits from several small meals of plankton-sized, protein-rich marine foods throughout the day.

• Active reef swimmer: Requires strong oxygenation, generous swimming room, secure retreats, and stable water quality.