Blackfin slatey

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.

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.

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).

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.

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.

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.

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.