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
This is a big Indo-West Pacific sweetlips/grunt that cruises reefs and hangs in caves, and it gets that cool yellow-and-silver look sprinkled with dark spots plus the really obvious black on the lower tail and the pelvic/anal fins. Juveniles show up in murkier estuary and silty reef areas, then the adults shift deeper and often sit in small groups until they go hunting at night. In aquariums its size is the whole story - it is a public-aquarium kind of fish once grown.
Also known as
Quick Facts
Size
45 cm SL
Temperament
Peaceful
Difficulty
Expert
Min Tank Size
300 gallons
Lifespan
10-20 years
Origin
Indo-West Pacific
Diet
Carnivore - meaty foods (shrimp, clam, squid, fish) and other benthic invertebrates; may eat small fish/crustaceans
Water Parameters
22-26°C
8.1-8.4
8-12 dGH
Need a heater for this species?
This species needs 22-26°C in a 300 gallon tank. Use our heater calculator to find the right wattage.
Calculate heater sizeCare Notes
- Plan for a big system - 300+ gallons with serious rockwork and caves, because they like to wedge themselves in and will claim a whole section as home.
- Keep salinity stable around 1.025-1.026 and run the tank like a nutrient-export machine (oversized skimmer, lots of flow), since they eat heavy and foul water fast.
- Feed meaty foods 1-2 times daily: shrimp, squid, clam, fish flesh, and quality frozen blends; soak in vitamins and rotate foods to dodge skinny-head/HLLE issues.
- They are a grouper-like ambush predator - anything that fits in its mouth turns into dinner, so skip small fish, tiny wrasses, and most decorative shrimp/crabs.
- Tankmates should be tough and similarly sized: big tangs, large angels, triggerfish, rabbitfish, and other robust predators; avoid other Diagramma and similar cave-loving bruisers unless the tank is huge.
- Quarantine is not optional - they ship rough and come in with flukes and bacterial mouth/fin rot, so watch for flashing, cloudy eyes, and frayed fins and treat early.
- Give them a tight lid and cover overflows - they spook and bolt, and a big fish in a weir box is a bad day for everyone.
- Breeding at home is basically a non-starter; they are likely offshore spawners with pelagic larvae, so just focus on keeping it fat, calm, and stable long term.
Compatibility
Good Tankmates
- Other mellow, similarly sized groupers and rock cod types that are not pushy at feeding time (think calm soapfish or smaller, laid-back grouper relatives) - they usually ignore each other if everyone has their own cave
- Peaceful reef-safe-ish tankmates that keep to themselves, like foxface rabbitfish - they are steady, not nippy, and not looking to pick fights
- Bigger, chill angels (Genicanthus angels are a great vibe) - they cruise the water column and do not constantly get in the slatey's face
- Medium to large tangs that are not total terrors (yellow, kole, tomini, etc.) - good activity without being predatory, just watch tang attitude in tight tanks
- Sturdy wrasses that are not tiny and not bully types (like a melanurus or a Halichoeres that can hold its own) - they move fast and do not get pinned in a corner
- Cardinals and other calm 'hover' fish ONLY if they are too big to fit in the slatey's mouth (big pajama/banggai groups in a large system can work) - size matters here
Avoid
- Tiny fish that look like snacks - small clownfish, small damsels, little gobies, firefish, cleaner wrasses - if it fits, it is food, especially at lights-out
- Hyper-territorial bullies like big maroon clowns, aggressive damsels, or dottybacks that live to harass rock-dwellers - they will stress the slatey and steal food
- Mean triggerfish (clown/queen/titan style) and really nasty large wrasses - they tend to turn the tank into a fight club and the slatey will take a beating or snap back hard
Where they come from
Blackfin slatey (Diagramma melanacrum) is a grunt from the Indo-Pacific, usually tied to coastal reefs, rocky areas, and lagoon edges. In the wild they cruise and hunt around structure, and a lot of their confidence comes from having caves and overhangs to tuck into when they feel watched.
They are not a common "reef display" fish for a reason: they get big, they eat like a predator, and they can be hard on tankmates if the setup is tight.
Setting up their tank
Think more like setting up for a big, messy predator than a dainty reef fish. Give them real swimming room, heavy filtration, and a rock layout that creates shade and bolt-holes. If they cannot claim a couple of safe spots, they stay jumpy and smash into things.
- Tank size: this is a large fish. I would not bother under 300 gallons, and bigger is easier long-term.
- Rockwork: build thick, stable caves and ledges. Use epoxy or rods if you stack rock - they hit rockwork hard during feeding.
- Flow: moderate to strong, but avoid blasting their favorite cave directly. Let them rest without getting pinned to the back wall.
- Filtration: big skimmer, lots of biological capacity, and a plan for nutrient export (water changes, refugium, algae scrubber, etc.).
- Cover: they can and do jump, especially new imports. A tight lid saves heartbreak.
Secure your rock. A startled slatey can bulldoze a flimsy pile, and once a rockslide happens you are chasing problems for months.
I like a bare-bottom or a very thin sandbed for these guys. They are powerful feeders and will keep a deep bed in constant sandstorm mode. If you run sand, go coarser so it does not blow everywhere.
What to feed them
They are enthusiastic carnivores and they learn your schedule fast. The trick is keeping them well-fed without turning the tank into a nitrate factory. Variety helps a lot, and so does feeding smaller portions more often instead of one giant meal.
- Staples: shrimp, squid, clam, mussel, and quality marine fish flesh (not oily freshwater stuff).
- Convenient foods: frozen predator blends and large meaty pellets once they recognize them as food.
- Treats: live blackworms or live shrimp can kickstart feeding on new arrivals (use sparingly and from a clean source).
Soak meaty foods in a vitamin supplement a couple times a week, especially early on. Big grunts can look fine while slowly developing deficiencies if you only feed one or two items.
Skip feeding whole silversides every day. It is a common habit with big predators and it tends to end in fatty fish and filthy water. Rotate foods, and if they will take pellets, use that to your advantage.
How they behave and who they get along with
They are bold once settled, but they are not "peaceful community" fish. Their main rule is simple: if it fits in their mouth, it is food. Even if it does not fit, they may still try a test bite.
- Best tankmates: other large, confident fish that can hold their own (bigger tangs, large angels, robust wrasses, some groupers of similar size).
- Avoid: small fish, slow hoverers, and anything shrimp/crab-like. Decorative crustaceans will not last.
- Corals: they are not coral eaters, but the "reef safe" question is mostly about collateral damage and hunting inverts.
Introduce them with a plan. If they are the last fish added to an established tank of big personalities, they often start out skittish and take longer to feed. If they are added early, they may claim the whole tank.
Aggression is usually about space and food. Give them line-of-sight breaks and feed in a way that spreads food around the tank. I have had the best results dropping food at two or three spots so one fish cannot guard the whole buffet.
Breeding tips
Breeding in home aquariums is basically a non-topic. These are open-water spawners in the wild and you would be dealing with a very large pair, seasonal cues, and pelagic larvae. Public-aquarium scale, not hobby scale.
If you ever see courtship behavior (circling, color shifts, late-day activity spikes), enjoy it for what it is, but do not count on raising larvae in a display system.
Common problems to watch for
Most issues with Blackfin slatey are the classic big-predator trio: shipping stress, parasites, and water quality. They can look tough and still crash fast if you skip quarantine or let ammonia spike.
- Refusing food after arrival: usually stress or parasites. Give dim lighting, a cave, and offer strong-smelling foods (clam, shrimp) without pestering them.
- Marine ich/velvet: these fish are not immune. A proper quarantine routine saves you from trying to catch a 12-20 inch fish out of a rock maze later.
- Bacterial mouth damage: they lunge hard at food and can scrape jaws on rock or acrylic. Watch for swelling or fuzzy edges and treat early.
- Nutrient creep: heavy feeding plus big biomass equals rising nitrate and phosphate. If algae is exploding, your slatey is probably being overfed or under-exported.
- HLLE-type erosion: often linked to diet gaps, stray voltage, and long-term water quality issues. Variety and clean water usually turn it around.
Do not "test" compatibility with small fish or shrimp. With this species, that experiment ends with a missing tankmate, usually overnight.
If you keep one successfully, you will notice the pattern: stable salinity, high oxygen, lots of structure, and a feeding routine that does not overwhelm your filtration. Nail those, and they are hardy in the long run - they just demand a serious system.
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