Yellow Assessor Basslet (Assessor flavissimus)

 

• Small fish safe: ✅ Yes

• Small invert safe: ✅ Yes

• 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 Yellow Assessor Basslet, also known as the Gold Assessor Basslet or Yellow Devilfish, boasts a vibrant yellow bodywith a subtle reddish-orange stripe extending from the eye to the upper edge of the gill cover. The dorsal and anal fins feature a submarginal reddish-orange band, adding to its striking appearance. This species is known 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 Central Pacific, particularly the Great Barrier Reef and Papua New Guinea, Assessor flavissimus inhabits reef environments at depths ranging from 5 to 20 meters (16 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 zooplankton and small crustaceans. 

 

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 – 27°C (72°F – 80°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, Yellow 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.