The marine betta, or comet, is a strikingly beautiful Indo-Pacific reef fish in the longfin family Plesiopidae, covered in white spots with a false eyespot on its rear dorsal fin, which it uses to mimic the head of a moray eel when threatened. Hardy, peaceful, and reef-safe, it is a prized but secretive aquarium centerpiece. (Despite the 'comet grouper' nickname it is not a true grouper.)
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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.
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Minimum
Peaceful FOWLR with caves
50 gal / 190 L cave-rich FOWLR
Calloplesiops altivelis is shy, nocturnal, and predatory on small fish/shrimp. Provide deep caves with overhangs (the comet morphology mimics a moray) and very peaceful tankmates.
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Recommended
Dim mature reef
75–90 gal / 280–340 L
More rockwork, more caves, lower light. Train onto frozen silversides/mysis. Avoid aggressive feeders (anthias, tangs) that will outcompete this slow night hunter.
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Ideal
Dedicated dim reef display
100 gal+ / 380 L+ dim display
Large dim-lit cave-heavy reef with carefully selected peaceful tankmates and reliable feeding. Marine bettas live 8–10+ years and display their comet mimicry fully in this environment.
Life & growth stages
How this animal changes through its life — each stage often has its own care, diet and space needs.
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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.
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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.
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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
Provide a 55+ gallon established marine aquarium with abundant live rock, caves, and overhangs; this is a shy, cave-dwelling species that needs plenty of hiding spots. Maintain reef parameters: 72-78 F (22-26 C), salinity 1.023-1.025 SG, pH 8.1-8.4, with moderate flow. A mature tank with stable chemistry suits them best.
Substrate
A standard live-sand reef substrate works well; the substrate itself is less important than abundant rockwork. Aquascape with plenty of caves, ledges, and dark recesses where the fish can retreat and ambush, since security greatly reduces stress.
Equipment & setup
A complete marine system: protein skimmer, reliable filtration, heater, and good live rock for biofiltration and structure. Moderate flow, stable reef-quality water, and a tight-fitting lid to prevent jumping. Subdued lighting or shaded caves encourage them to show themselves more.
Diet
Carnivorous ambush predator. Feed meaty marine foods: frozen mysis, chopped shrimp and seafood, and enriched marine pellets once weaned. Wild-caught individuals may initially need live foods (ghost shrimp, live mysis) before transitioning to frozen. Will eat small fish and shrimp that fit in their large mouth, so choose tankmates accordingly. Feed in the evening when they are most active.
Behavior & temperament
Peaceful, slow-moving, and secretive — largely nocturnal and reclusive, often hiding by day. Its famous defense is reversing into a crevice and flaring its fins so the dorsal eyespot mimics a moray eel's face. Reef-safe with corals but will eat small fish, shrimp, and ornamental crustaceans. Keep one per tank; conspecifics may quarrel except as a confirmed pair. Not handleable.
Health
Generally hardy once established and feeding. The main challenge is getting newly imported wild fish to eat; many are net- or even cyanide-caught, so source captive-bred (now widely available) for reliability and conservation. Susceptible to marine ich and velvet like other reef fish; quarantine before adding to a display. A secure lid is essential as they can jump.
Tips, DIY & hacks
Buy captive-bred comets where possible — they wean onto frozen food easily and avoid the cyanide-caught wild trade. Provide ample caves so it feels secure and ventures out more. Do not house with bite-sized fish, shrimp, or tiny ornamental crabs, which it will eat. Target-feed in the evening until it learns to take prepared foods confidently.
