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
Diet
Carnivore
Difficulty
Easy
Breeding
Mouthbrooder · 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

The blue assessor is a small, peaceful fish for a reef tank with plenty of rockwork. It is easy to keep.

Did you know

Blue assessors can shelter a hundred strong in a single wild cave, all hovering belly-up against the ceiling at once. That scale is nearly impossible to recreate at home, though even a small group under one ledge makes the same upside-down show.

Caring for the blue assessor

Give blue assessors a tank of at least 29 gal (110 L), with caves and overhangs built in before the fish is added. Plan on considerably more room if you keep several. They spend their time hovering upside down under a favorite ledge rather than patrolling the tank.

Keep the water at 72 to 79°F (22 to 26°C) and a pH of 8.1 to 8.4. Feed a varied carnivore diet of mysis, finely chopped seafood and enriched brine shrimp at least twice a day.

Blue assessors are peaceful toward other species and will not touch corals or invertebrates, so they make excellent reef tank mates. They are more particular about their own kind. A single fish is the simplest choice, since adults introduced separately will often fight over territory. A group added at the same time can settle into a stable hierarchy in a large enough aquarium.

The male carries the fertilized eggs in his mouth for around two weeks. As they near hatching, he keeps his mouth open and shuffles the eggs back and forth every few minutes to keep them aerated. Breeding them is moderately hard.

Good to know

Assessor macneilli belongs to the family Plesiopidae and is the classic mouthbrooder of its small genus. It is not produced at the same commercial scale as some dottybacks, but captive-bred fish from farms such as ORA reach the trade.

In the wild

The blue assessor lives in the western Pacific, from northeastern Australia and New Guinea to New Caledonia. Its range takes in the Great Barrier Reef and the Coral Sea. It shelters in caves and under ledges in shallow water, down to about 20 meters.

Weekly picks

Tank mates

The 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.

What does a blue assessor eat?

A blue assessor eats a varied carnivore diet of mysis, finely chopped seafood and enriched brine shrimp. Feed it at least twice a day.

How big does a blue assessor get?

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

How does a blue assessor breed?

Blue assessors are mouthbrooders. The male carries the fertilized eggs in his mouth for around two weeks, shuffling them every few minutes near hatching to keep them aerated. Breeding them is moderately hard.

Related species