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Yellow Assessor

Assessor flavissimus

The yellow assessor is a strikingly bright, peaceful reef inhabitant that frequently swims upside down under ledges. It requires calm tankmates and extensive, shaded rockwork to feel fully secure.

Taxonomy

Class:Actinopteri

Order:Perciformes

Family:Plesiopidae

Genus:Assessor

Species:Assessor flavissimus

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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 species is native to the Western Central Pacific, particularly common along the Great Barrier Reef in Australia. It inhabits seaward reef slopes and vertical drop-offs at depths of 5 to 20 meters, living in loose schools inside dark caves and under ledges.

Taxonomy and Morphology:

Member of the family Plesiopidae. Assessor flavissimus features an elongated, laterally compressed body with a deeply forked caudal fin. It grows up to 7 cm in length and has large eyes. It has a unique physiological adaptation allowing it to swim upside down.

Social Behavior:

Extremely peaceful, shy, and non-aggressive. In nature, it forms large colonies. In the aquarium, it can be kept individually or in small groups if introduced at the same time. It will not harass any tankmates and is easily intimidated by active fish.

Coloration and Sexual Dimorphism:

The entire body displays a solid, brilliant lemon-to-golden yellow coloration. It features thin orange-red margins along the dorsal and anal fins, and a thin blue-violet ring around the iris of its eyes. No visible sexual dimorphism exists.

Care, breeding and tankmates

Aquarium Setup:

Requires a tank of at least 100 liters with plenty of live rock arranged to form caves, overhangs, and arches. Shaded areas and dim lighting will encourage it to swim in the open. The tank must be tightly covered as they may jump if startled.

Diet and Feeding:

Carnivorous planktivore. In captivity, feed high-quality frozen foods like mysis, enriched brine shrimp, copepods, and finely minced seafood. Over time, it will accept dry flakes and pellets. Feed twice daily in a quiet area of the tank.

Water Quality:

Requires standard marine reef conditions: temperature at 22-27°C, pH at 8.1-8.4, salinity at SG 1.020-1.025, and KH between 8 and 12 dKH. Ammonia and nitrite must be kept at zero, with low nitrates and moderate water flow.

Compatibility and Cohabitation:

100% reef safe. It is completely safe with all corals and invertebrates, including small shrimp. Keep with peaceful tankmates such as firefish, gobies, cardinals, and peaceful clownfish. Do not house with aggressive or fast-swimming species.

Aquarium Breeding:

Breeding has been achieved in captivity. They are paternal mouthbrooders. After spawning, the male carries a spherical egg mass in his mouth for approximately two weeks until hatching, during which time he does not feed.

Risks and Diseases:

The main risk is starvation due to stress from aggressive tankmates, which will cause it to hide constantly. Susceptible to marine ich (Cryptocaryon) under stress. Slow acclimation and a calm environment are vital for its success.

Geographic Origin and Distribution

Origin

Tropical oceans and reefsAustralia, 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 = 0.00468, b = 3.18

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.

Blue assessor

Assessor macneilli

Species description

Care, breeding and tankmates