Yellow Assessor

Assessor flavissimus

Also known as Yellow Assessor Basslet, Yellow Devilfish, Gold Assessor Basslet

SaltwaterFish

Yellow Assessor (Assessor flavissimus) — jpdandretta · CC BY 4.0

Care at a glance

Temperature: 72–79°F (22–26°C)
pH: 8.1–8.4
Adult size: 2.8 in (7 cm)
Minimum tank: 29 gal (110 L)
Temperament: Peaceful
Social behavior: Social
Swim level: Middle
Difficulty: Easy
Breeding: Medium
Rarity: Rare
Reef safe: Reef safe
Origin: Western Pacific, around the Great Barrier Reef, the Coral Sea, and Papua New Guinea
Family: Plesiopidae

Overview

A genuinely peaceful plankton picker that gets along with nearly everything except its own kind in a cramped tank, where it can turn snappish over territory. It is completely reef safe and will not bother corals or invertebrates. Because it hovers upside down under a ledge rather than swimming freely through the tank, the aquascape needs to be built around it, with caves and overhangs in place before the fish goes in. It feeds on small zooplankton in the wild and readily accepts frozen mysis, enriched brine shrimp, and other small meaty foods in the aquarium.

In the wild

Found on the Great Barrier Reef, the Coral Sea, and reefs off Papua New Guinea, sheltering in small groups under overhangs, ledges, and inside caves. It swims upside down against the ceiling of its shelter, oriented to the rock rather than to the surface, and rarely ventures far from cover. Rockwork is the single most important thing to get right: without real caves and overhangs to hide under and swim against, this fish stays stressed and loses its color. A tank of at least 110 liters gives it room for a cave system, with 200 liters or more recommended for keeping a small group.

Good to know

Assessor flavissimus belongs to the small reef family Plesiopidae. Males are mouthbrooders, holding the fertilized eggs in their mouths until they hatch rather than leaving them in a nest. It is bred commercially in modest numbers, notably by ORA, so tank-raised individuals are available alongside wild-caught fish.

Did you know

It spends nearly all of its time upside down, cruising along the underside of a ledge with its belly toward the rock and its back toward open water, an orientation so consistent that a yellow assessor kept without an overhang to swim against will struggle to feel secure.