Squareback Anthias - Male - Fiji

The male Squareback Anthias (Pseudanthias pleurotaenia), also known as the Squarespot Anthias, Square-Spot Fairy Basslet, or Purple Blotch Anthias, is a spectacular saltwater fish recognized for its vivid pink, reddish-orange, and magenta coloration. Mature males typically develop a large pale lavender-to-whitish quadrangular patch on the side of the body, along with blue-edged fins, an elongated dorsal spine, and a deeply forked tail. The size and intensity of the characteristic flank patch can vary between individuals and geographic populations.

This Fiji-origin form comes from the South Pacific portion of the species’ range, where Squareback Anthias inhabit current-swept reef slopes and drop-offs. In nature, they form aggregations several feet above reef structure, remaining in the water column to capture passing zooplankton while retreating quickly toward the reef when threatened.

Reaching approximately 8 inches as an adult, the male Squareback Anthias is substantially larger than many commonly maintained anthias and requires generous swimming space. Its dramatic coloration, active behavior, and coral-safe feeding habits make it an impressive centerpiece for a spacious reef aquarium or appropriately sized fish-only aquarium.

Quick Care Guide

Care Level: Moderate
Minimum Tank Size: 125 gallons
Temperament: Semi-aggressive
Diet: Carnivore / Planktivore
Reef Safe: Yes
Region: Fiji – South 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, so a long aquarium footprint with substantial unobstructed swimming room is preferable for long-term housing.

Arrange live rock to provide caves, ledges, overhangs, and shaded retreats without excessively reducing open water. Secure retreats are important for nighttime shelter and allow the fish to withdraw from aggressive interactions or bright aquarium lighting when desired.

Moderate to strong, well-distributed water movement is ideal and helps replicate the current-swept reef habitats where this species naturally feeds. Strong oxygenation and efficient filtration are especially important because frequent feeding can contribute significant 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.

Provide two to three small meals per day, with additional smaller feedings beneficial when practical. Newly introduced specimens may initially respond most readily to mysis shrimp, copepods, and other frozen planktonic foods before becoming accustomed to 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 females, or aggressively challenge competing males as they establish and maintain social dominance.

Suitable tankmates include tangs, rabbitfish, peaceful to moderately assertive angelfish, fairy and flasher wrasses, clownfish, gobies, and other non-predatory marine species 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 system.

Like other anthias, this species is a protogynous hermaphrodite. If a dominant male is absent, a dominant female may transition into a male as the social structure of the group changes.

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 will 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 anthias 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 typically develop vivid pink-to-magenta coloration with a prominent pale square or rectangular flank patch.

• Male territoriality: May aggressively challenge other male anthias and chase subordinate individuals.

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

• Active reef swimmer: Requires strong oxygenation, open water, secure retreats, and a mature aquarium environment.