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Blue assessor

Assessor macneilli

The blue assessor is a fascinating cave-dweller known to swim completely upside down. With its dark navy-blue body, this peaceful carnivore thrives in small reef tanks loaded with dark overhangs.

Taxonomy

Class:Actinopteri

Order:Perciformes

Family:Plesiopidae

Genus:Assessor

Species:Assessor macneilli

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Water Temperature

22 - 27 °C

pH Value

8.1 - 8.4

Water type

Marine

Tank level

All levels

Adult size

7 cm

Minimum tank volume

>= 100 L

Minimum group

>= 1

8 - 12 dKH

Species description

Geographical Origin and Habitat:

This fish lives along the Great Barrier Reef in Australia and the Coral Sea. It inhabits deep outer reefs, finding shelter in dark caves, crevices, and shaded vertical walls.

Taxonomy and Morphology:

Belongs to the Plesiopidae family. It features an elongated, laterally compressed body with large eyes adapted to low light. It reaches a maximum length of 6-7 cm when fully grown.

Social Behavior:

A very peaceful, shy, and cryptic species that lives in small groups inside rocky caves. It does not bother corals or other fish, making it ideal for quiet reef aquariums.

Coloration and Sexual Dimorphism:

The coloration is a solid, deep navy blue covering the entire body. The fins are edged with brilliant electric blue lines. No obvious external sexual dimorphism is present.

Care, breeding and tankmates

Aquarium Setup:

Requires a tank of at least 80-100 liters with plenty of live rock forming arches, deep caves, and shaded zones. A lid is mandatory to prevent the fish from jumping out.

Diet and Feeding:

Feeds on zooplankton. In the aquarium, it accepts frozen mysis, brine shrimp, and lobster eggs. Once acclimated, it consumes high-quality micro-granules fed near its cave.

Water Quality:

Keep marine parameters stable: temperature 22-26 °C, pH 8.1-8.4, and specific gravity 1.020-1.025. Low nitrates and an efficient protein skimmer are required.

Compatibility and Cohabitation:

Compatible with corals and benthic invertebrates. Can live with peaceful fish like gobies, cardinalfish, and small wrasses. Avoid aggressive or fast-swimming tankmates.

Aquarium Breeding:

They are paternal mouthbrooders. The male carries and incubates the egg mass in its mouth for about 10-15 days until hatching. Fry must be fed with rotifers.

Risks and Diseases:

Sensitive to acclimation stress and starvation if kept with overly voracious fish. Acclimate slowly and ensure it gets enough food near its hiding spot.

Geographic Origin and Distribution

This fish lives along the Great Barrier Reef in Australia and the Coral Sea. It inhabits deep outer reefs, finding shelter in dark caves, crevices, and shaded vertical walls.

Origin

Tropical oceans and reefsEast AsiaAustralia, New Guinea, and Oceania

Fish profile

Temperament: Peaceful
Diet: Carnivore
Sex ratio: Group
Feeding frequency: Twice daily
Bioload: Assessors have a very low bioload. They are small and do not produce much waste. Perfect for smaller reef tanks.
Flow: Strong
Jump risk: Yes
Reproduction: Mouthbrooder
Compatibility & tankmates: Reef safe
Tank level: All levels

FishBase

Window to the official scientific data of the species

Data synced on: 06/15/2026

Scientific Parameters & Models

Trophic level: 3.4 ± 0.2 se
Resilience: High (less than 15 months)
Phylogenetic uniqueness (PD50): 0.625
Bayesian Length-Weight: a = 0.01047, b = 2.89

Image gallery

Licensed images linked to the species or, when marked, to the closest representative taxon.

Related species

Recommendations grouped by close relatives, companion choices, and similar care.

Variants and close relatives

Same atlas type, nearby scientific identity, genus, or family.

Yellow Assessor

Assessor flavissimus

Species description

Care, breeding and tankmates