Blue Assessor Basslet (Assessor macneilli)

 

• Small fish safe: ✅ Yes

• Small invert safe: ⚠️ With caution (may prey on very small ornamental shrimps)

• Coral safe: ✅ Yes

• Minimum tank size: 114 liters (30 gallons)

• Adult size: Up to 7.6 cm (3 inches)

• Diet: Carnivorous, feeding on small crustaceans and zooplankton

• Be aware of: Prefers peaceful tank mates; requires ample hiding spots; may swim upside down under ledges

 

Appearance and Coloration

The Blue Assessor Basslet, also known as the Blue Scissortail or Blue Devilfish, features a deep blue body with a subtle pale blue margin along the dorsal, anal, and pelvic fins. This species is noted for its unique behavior of swimming upside down under ledges and in caves, aligning its belly to the reef surface. 

 

Natural Habitat and Behavior

Native to the Western Pacific, particularly the Great Barrier Reef in Australia, Assessor macneilli inhabits reef environments at depths ranging from 2 to 20 meters (6.5 to 65 feet). They are often found in caves and under ledges, exhibiting their characteristic upside-down swimming behavior. In the wild, they are planktivores, feeding on small crustaceans such as copepods, ostracods, and amphipods. 

 

Tank Requirements and Water Parameters

• Minimum Tank Size: 30 gallons (114 liters) for a single specimen; larger tanks are recommended for multiple individuals.

• Water Parameters:

• Temperature: 22°C – 26°C (72°F – 78°F)

• pH: 8.1 – 8.4

• Specific Gravity: 1.020 – 1.025

• Aquascaping:

• Provide ample hiding spots using live rock formations, caves, and overhangs to mimic their natural habitat.

• Ensure a mix of open swimming areas and sheltered spaces to accommodate their unique swimming behavior.

• Lighting: Moderate; subdued lighting can help reduce stress, especially during acclimation.

• Water Movement: Moderate flow, replicating natural reef conditions.

 

Diet and Feeding

In their natural environment, Blue Assessor Basslets primarily consume zooplankton and small crustaceans. In captivity:

• Captive Diet:

• Mysis shrimp

• Brine shrimp

• Finely chopped seafood (e.g., shrimp, squid)

• High-quality marine pellets and flakes

• Feeding Frequency: At least twice daily to maintain health and vibrant coloration. 

 

Compatibility and Social Structure

• Temperament:

• Generally peaceful, making them suitable for community tanks.

• May exhibit territorial behavior toward conspecifics or similar species if not introduced simultaneously.

• Tank Mates:

• Compatible with other peaceful fish such as gobies, clownfish, and small wrasses.

• Avoid aggressive species that may intimidate or outcompete them.

• Can be kept in groups if introduced at the same time; a larger tank is recommended to provide sufficient territory. 

• Reef Compatibility: Reef-safe; does not harm corals or invertebrates.

 

Important Considerations

• Behavior: Known for their unique habit of swimming upside down under ledges and in caves.

• Hiding Spots: Providing ample hiding places will help reduce stress and allow the fish to exhibit natural behaviors.

• Tank Mates: Due to their docile nature, they should not be housed with aggressive species that may cause them to hide excessively.

• Acclimation: May be shy upon introduction; providing a peaceful environment with plenty of hiding spots can help them adjust.