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Assessor flavissimus, Yellow Assessor. Scott W. Michael

Family: Plesiopidae

Species in Genus Assessor:

Reference: A PocketExpert Guide to Reef Aquarium Fishes
Image credit: SWM
Text credit: SWM


This outstanding group of stunningly colored fishes is a welcome addition to virtually all reasonably peaceful reef aquariums. Two species are most commonly available: the Yellow Assessor (Assessor flavissimus) and the Blue Assessor (Assessor mcneilli). Both live in caves, under overhangs, or in large crevices, where they typically swim upside down.

The Yellow Assessor usually occurs singly, in pairs, or in small groups. The Blue Assessor is more often found in large groups. Both feed on zooplankton and are very hardy.

To keep more than one in your reef tank, be sure the tank is large enough (100 gallons [380 L] or more) with ample hiding spaces. Add the pair or small group members all at once. It is also better if the fish differ in size to increase the chances of getting both sexes. Males are larger and more likely to fight.

It is best not to include them in a tank with overly boisterous species as they are likely to be the targets of aggression. Neither species will bother ornamental invertebrates. Both reach a length of just over 2 in. (5.1 cm). They are ideally suited for the nano-reef aquarium.