The yellow assessor is a small, brilliant golden-yellow reef fish in the longfin family Plesiopidae, native to the Western Pacific, famed for swimming upside-down beneath ledges and inside caves. Peaceful, hardy, and reef-safe, it is one of the best nano and reef centerpieces and is now widely captive-bred (notably by ORA).
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Western Pacific — including the Great Barrier Reef and Coral Sea (Australia), Fiji and Tonga
Origin
Old World
Climate
🌴 Tropical
Water type
🌊 Marine
Family
Plesiopidae
Genus
Assessor
Part of the Dottybacks & Basslets
Small, colorful cave- and ledge-dwelling marine fish popular as hardy reef and nano centerpieces, including the dottybacks (Pseudochromidae) and the longfin basslets and assessors (Plesiopidae). Most are reef-safe, secretive, and prone to jumping, so a covered, rock-rich tank suits them best.
From the minimum an animal needs to be kept humanely, up to the ideal setup. Bigger is almost always better — minimums are floors, not targets.
Photo coming soon
Minimum
Cave-rich nano reef
30 gal / 110 L reef
Assessor flavissimus is a peaceful cave-dweller that swims inverted under overhangs. Caves and ledges essential, peaceful tankmates, gentle flow.
Photo coming soon
Recommended
Mature reef with overhangs
55 gal / 200 L+
Larger reef with abundant rock overhangs and peaceful tankmates. Trios (1 male, 2 females) work in larger systems. Reef-safe and long-lived.
Photo coming soon
Ideal
Mature mixed reef
75 gal+ / 280 L+ mixed reef
Spacious reef with complex rockwork and varied lighting zones. Small harem displays full yellow colour and natural inverted swimming under overhangs.
Life & growth stages
How this animal changes through its life — each stage often has its own care, diet and space needs.
Photo coming soon
Egg
Fish eggs are small, translucent spheres, often laid in clutches on plants, substrate, or in a nest — or carried/brooded by a parent in livebearing and mouth-brooding species. A dark eye spot and the curled embryo become visible inside as development progresses.
Photo coming soon
Fry
Newly hatched fry are tiny and semi-transparent, frequently still carrying a yolk sac that fuels them before they feed freely. They lack full fin structure and adult coloration, staying near cover until they can swim and forage on their own.
Photo coming soon
Juvenile
Juveniles look like miniature adults but with developing fins and muted or different markings; many species shift pattern and color as they mature. Growth is rapid at this stage given clean water and steady feeding.
Adult
Adults show the species' full size, finnage, and mature coloration, and are sexually mature. Many fish develop sex-specific differences in size, color, or fin shape, which can intensify during breeding.
Habitat & enclosure
A 30+ gallon established marine/reef aquarium with abundant live rock arranged into caves, overhangs, and ledges suits them; this is a cave- and ledge-orienting species that often hovers upside-down under rock. Maintain reef parameters: 72-78 F (22-26 C), salinity 1.023-1.025 SG, pH 8.1-8.4, with low to moderate flow. A mature, stable tank with plenty of shaded recesses brings out their best color and confidence. Allow 50+ gallons with multiple separated cave systems if keeping a small group.
Substrate
A standard live-sand reef substrate works fine; structure matters more than substrate. Aquascape with plenty of caves, ledges, and overhangs where the fish can hover and retreat, since secure shaded spots greatly reduce stress and encourage natural upside-down behavior.
Equipment & setup
A complete nano/reef marine system: heater, reliable filtration, protein skimmer (recommended), and quality live rock for biofiltration and structure. Low to moderate flow and stable reef-quality water. A tight, fully-covered lid is critical to prevent jumping. Subdued or shaded lighting in cave areas encourages them to display out in the open.
Diet
Carnivore/zooplanktivore. Feed small meaty marine foods: frozen mysis, enriched brine, finely chopped seafood, and small marine pellets/flakes once weaned. Offer several small feedings a day for nano specimens. Captive-bred individuals readily take prepared foods; wild fish may need live brine or mysis to start before transitioning to frozen.
Behavior & temperament
Peaceful, somewhat shy, and inoffensive toward other species, but can be semi-aggressive toward conspecifics in cramped tanks. Does best in a calm tank without aggressive or boisterous tankmates that outcompete it for food. Reef-safe with corals and ornamental invertebrates. Frequently swims and rests upside-down under ledges and in caves. Can be kept singly, as a pair, or in a small group in a larger tank with ample separated caves; provide enough hiding spots to spread out and reduce squabbling. A mouth-brooding species (males incubate eggs). Not a handleable fish.
Health
Generally hardy and disease-resistant once established and feeding. Susceptible to marine ich and velvet like other reef fish, so quarantine new arrivals. The main risks are starvation in competitive tanks and jumping — a tight-fitting lid is essential, as assessors are accomplished jumpers, especially when startled or newly introduced. Choose captive-bred stock for the easiest acclimation and feeding.
Tips, DIY & hacks
Buy captive-bred yellow assessors where possible — they wean onto frozen and prepared foods easily and acclimate reliably. Cover the tank completely, as they jump readily in the first weeks. Provide multiple caves and ledges so the fish feels secure and ventures out. Pair with peaceful tankmates so this gentle fish is not outcompeted at feeding time; target-feed if needed until it feeds confidently.
