Blue Assessor (Assessor macneilli)

The Blue Assessor, often called the Blue Scissortail or Blue Devilfish, is the moody, sapphire-toned cousin of the Yellow Assessor. Just like its yellow relative, it is a member of the Plesiopidae (Longfin) family and is famous for its gravity-defying habit of swimming upside down under ledges and cave ceilings. While it shares many traits with the yellow version, the Blue Assessor is a mouthbrooder (the male carries the eggs), a fascinating biological quirk that sets it apart.

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Key Information

• Common Name: Blue Assessor, Macneill’s Assessor, Blue Scissortail

• Scientific Name: Assessor macneilli

• Origin: Western Pacific (Great Barrier Reef, Coral Sea, New Caledonia)

• Max Size: 6–7.5 cm (approx. 2.5–3 inches)

• Lifespan: 5–8 years

• Temperament: Peaceful but territorial toward other basslets

• Diet: Carnivore – focuses on zooplankton like mysis, brine shrimp, and copepods

• Care Level: Easy to Moderate

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Water Parameters

• Temperature: 22–27°C (72–80°F)

• pH: 7.8–8.5

• Specific Gravity: 1.020–1.026

• Minimum Tank Size: 115L (30 Gallons) for a single; 210L+ (55+ Gallons) for a group

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Behaviour & Tank Compatibility

The Blue Assessor is a calm, cryptic fish that brings a "mystical" vibe to the reef. It is less visible than many open-water swimmers but adds incredible detail to the shaded parts of your tank.

• The "Shadow Dweller": They are shy and prefer dimly lit areas. They may take time to feel comfortable venturing out into high-intensity reef lighting.

• Territoriality: While peaceful toward most tank mates, they do not tolerate other basslets or dottybacks well. Keep only one per tank unless you have a large system and introduce a group simultaneously.

• Tank Mates: Best kept with other peaceful reef fish like Clownfish, Cardinals, Blennies, and Pipefish. Avoid aggressive "bullies" like large Damsels or Tangs that might harass them.

• Reef Safe: 100% Reef Safe. They are a perfect choice for coral-heavy tanks as they ignore all sessile invertebrates.

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Ideal Tank Setup

• Rockwork is Key: You must provide caves and overhangs. Without a "ceiling" to swim under, they will be constantly stressed.

• Lighting: If you have a high-PAR setup for corals, ensure there are several deep shaded zones where the Assessor can hang out during the day.

• Lid: Like its yellow cousin, the Blue Assessor is a notorious jumper. A tight-fitting lid or mesh top is non-negotiable.

• Flow: They are not the strongest swimmers; moderate water flow is ideal. Avoid putting them directly in the path of high-powered wavemakers.

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Why Choose the Blue Assessor?

The Blue Assessor is for the hobbyist who loves deep, moody colors and unique evolutionary traits. Its dark navy body with electric blue fin margins is incredibly striking, especially when it peeks out from a dark cave. If you are already considering the Yellow Assessor, adding a Blue one to a different (or sufficiently large) system gives you a complete look at one of the reef's most interesting "upside-down" families.