Blackfin slatey
Diagramma melanacrum
The Blackfin slatey features a streamlined body, dark blue-gray coloration, and distinctive black fins, particularly prominent in adults.
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About the Blackfin slatey
A large sweetlips from Southeast Asian reefs with a pale silvery-yellow body sprinkled in dark spots and distinct black pelvic and anal fins. Juveniles linger in silty shallows, while adults rest in small groups on deeper reef walls by day and roam at night to pick off crabs, shrimp, and other invertebrates.
Also known as
Quick Facts
Size
50 cm TL
Temperament
Peaceful
Difficulty
Intermediate
Min Tank Size
240 gallons
Lifespan
10-20 years
Origin
Southeast Asia and Timor Sea (Indo-West Pacific)
Diet
Carnivore - meaty foods such as shrimp, crabs, worms, and small fish
Water Parameters
21-28°C
8-8.5
Need a heater for this species?
This species needs 21-28°C in a 240 gallon tank. Use our heater calculator to find the right wattage.
Calculate heater sizeCare Notes
- Start with at least 240 gallons and a long footprint; Blackfin slatey reaches about 50 cm and lives 10-20 years, so plan long term.
- Stack sturdy rock to form large caves and shaded overhangs; adults rest in dim spots by day and roam more at night.
- Maintain 21-28 C and pH 8.0-8.5 in a stable marine system; a big carnivore means heavy waste, so oversize filtration and keep up on water changes.
- Feed a varied meaty diet: shrimp, crabs, worms, and small fish; offering food at dusk or after lights out often gets a stronger response.
- Peaceful but predatory; ornamental crustaceans and very small fish are likely meals, so choose tankmates that are calm yet too large to swallow.
- Juveniles come from silty shallows; giving youngsters quiet, sheltered areas helps them settle.
- Wild adults rest in small groups, but housing multiples takes enormous space; most aquarists keep a single specimen.
- Captive breeding is not documented for this species; do not expect pairing or sexing in home aquaria.
Compatibility
Good Tankmates
- Large, peaceful tangs (Zebrasoma, Ctenochaetus, Naso) in very large tanks - avoid the hyper-aggressive species
- Adult rabbitfish/foxfaces (Siganus spp.) - calm grazers that generally ignore sweetlips
- Peaceful swallowtail angelfish (Genicanthus spp.) and other calm, similarly sized angelfish
- Robust, peaceful butterflyfish and Heniochus bannerfish that are not fin-nippers
- Platax batfish - large, placid open-water swimmers suited to very large systems
- Large squirrelfish and soldierfish (Holocentridae) that are too big to be swallowed
Avoid
- Ornamental shrimp, crabs, and other motile invertebrates - will be hunted and eaten
- Small or slender fish that can be swallowed (chromis, gobies, firefish, anthias, small wrasses)
- Aggressive or fin-nipping fishes (triggers, large Thalassoma wrasses, damsels; hyper-aggressive tangs like Sohal or Clown tang)
- Large predatory fishes (groupers, big moray eels, snappers) that may harass or outcompete
The blackfin slatey (Diagramma melanacrum) is a large, peacefully disposed sweetlips from the Indo-West Pacific. Adults show a pale silvery-yellow body dusted with dark spots and distinct black pelvic and anal fins. It reaches up to 50 cm and can live 10-20 years, so plan for a long-term, big-fish commitment.
Origin and natural habitat
This marine species occurs in Southeast Asia and the Timor Sea. Juveniles linger in silty shallows, while adults rest by day in small groups on deeper reef walls. After dark they roam to pick off crabs, shrimp, worms, and other invertebrates.
Aquarium setup and water conditions
A single adult needs a very large system: at least 240 US gallons. Provide a stable marine environment at 21-28 C with pH 8.0-8.5. Build rockwork with ledges and caves so the fish can tuck away during the day, and leave open stretches for cruising at night. Subdued lighting or shaded zones help mimic the deeper reef feel. Because this is a big carnivore that produces a lot of waste, use robust filtration and good aeration.
Not reef safe with mobile invertebrates: this species eats shrimp, crabs, and worms, and may take small fish.
Diet and feeding
A strict carnivore. Offer meaty marine foods such as shrimp, pieces of crab, marine worms, and small fish or fish flesh. Provide appropriately sized portions. Feeding later in the day or after lights dim often suits their natural nocturnal foraging rhythm.
Behaviour and tankmates
Blackfin slateys are peaceful and spend much of the day resting, becoming more active at night. Choose tankmates that are similarly non-aggressive and too large to be viewed as prey. Avoid small ornamental fish and invertebrates, which are part of this species' natural diet.
Common pitfalls to avoid
- Underestimating adult size: plan around a 50 cm fish and a minimum 240-gallon tank with ample swimming room.
- Expecting a clean-up crew: shrimp, crabs, and worms will be eaten.
- Bright, exposed aquascapes: lack of caves and shaded areas can stress a fish that rests by day.
- Letting water quality slide: heavy, meaty feeding and a large body mean high waste production; use strong filtration and keep up with maintenance.
- Mixing with bullies: highly aggressive species can harass this calm fish.
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