TankBud

Blue assessor

Assessor macneilli

Also known as Blue Devilfish, Blue Scissortail, Macneill's Assessor

SaltwaterFish

Blue assessor (Assessor macneilli)
Rick Stuart-Smith / Reef Life Survey · CC BY 3.0

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
Family
Plesiopidae
Origin
Western Pacific, from northeastern Australia and New Guinea through the Great Barrier Reef and Coral Sea to New Caledonia

Overview

Blue assessors are peaceful toward other species and spend their time hovering upside down under a favorite ledge rather than patrolling the tank. They are more particular about their own kind: a single fish is the simplest way to keep one, since adults introduced separately will often fight over territory, though a group added to the tank at the same time can settle into a stable hierarchy in a large enough aquarium. They are excellent reef tankmates and will not touch corals or invertebrates. Feed a varied carnivore diet of mysis, finely chopped seafood, and enriched brine shrimp at least twice a day.

In the wild

Ranges across the western Pacific from northeastern Australia and New Guinea through the Great Barrier Reef and Coral Sea to New Caledonia, sheltering in caves and under ledges in shallow water down to about 20 meters. Wild groups can number up to a hundred fish packed into a single cave system, all swimming upside down against the roof. In the aquarium it needs the same setup on a smaller scale: a tank of at least 110 liters with caves and overhangs built in before the fish is added, and considerably more if keeping several.

Good to know

Assessor macneilli belongs to the family Plesiopidae and is the classic mouthbrooding member of its small genus: the male carries the fertilized eggs in his mouth for around two weeks, and as they near hatching he keeps his mouth open and shuffles the eggs back and forth every few minutes to keep them aerated. It has not been produced at the same commercial scale as some dottybacks, though captive-bred individuals from farms such as ORA do reach the trade.

Did you know

In the wild a single cave can shelter a shoal of a hundred blue assessors all hovering belly-up against the ceiling at once, a striking sight that is nearly impossible to recreate at that scale in a home aquarium, though even a small group under one ledge captures the same upside-down spectacle.

Weekly picks

Tank mates

Blue assessor is a peaceful, social species that keeps to the middle. These picks share a community profile with it, are safe together in both directions, and overlap on water type, temperature and pH.

Check the full mix in the stocking calculator

Common questions

What size tank does a Blue assessor need?

A Blue assessor needs at least 29 gal (110 L). It spends most of its time in the middle.

What temperature and pH does a Blue assessor need?

A Blue assessor is comfortable at 72–79°F (22–26°C) with a pH of 8.1–8.4.

How big does a Blue assessor get?

An adult Blue assessor reaches about 2.8 in (7 cm).

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