McCosker's coralbrotula

Where they come from

McCosker's coralbrotula (Ogilbia mccoskeri) is one of those little reef oddballs that most people never see because it does not want to be seen. They come from coral reef rubble and crevices in the tropical western Atlantic/Caribbean region, spending most of their lives wedged into dark holes. Think "tiny secretive cave fish" more than "open-water swimmer."

Setting up their tank

The whole game with coralbrotulas is giving them a place where they feel safe enough to settle in. If they feel exposed, they will either vanish forever (in the rockwork) or slowly waste away from stress and missed meals.

• Tank size: I would not bother below 20 gallons, and 30+ makes everything easier (stable salinity, more hiding options, less "where did it go" panic).
• Rockwork: build a rock pile with lots of tight cracks, caves, and rubble zones. I like adding a few small chunks of coral skeleton or broken rock in a corner so they can choose micro-hides.
• Substrate: sand is fine. A rubble patch (small shells/rock bits) is even better for that natural crevice feel.
• Flow and light: moderate flow is fine, but make sure there are calm pockets behind rocks. Bright lighting is OK if you create real shade.
• Cover: use a lid. These are eel-ish little fish and can surprise you.
• Filtration: keep nutrients reasonable, but do not run the tank sterile. A mature reef with pods and microfauna helps a lot.

Acclimation matters more than people expect. Slow drip, lights low, and give them a quiet first week. If you rearrange rockwork a bunch early on, they can take it personally and go off food.

What to feed them

Mine did best on small meaty foods delivered after the lights were dim. They are micro-predators, not algae grazers. If you only feed big chunks once a day in full daylight, you will mostly be feeding your cleanup crew.

• Good staples: mysis (smaller pieces), finely chopped shrimp, chopped clam, enriched brine (as a treat, not the main diet).
• Best "get them started" foods: live or fresh-hatched stuff that moves (live brine, small live mysids if you can get them), and tiny bits of raw seafood wiggled with forceps.
• Pellets: some individuals will take small sinking pellets, but do not count on it. Use them as backup, not the plan.
• Feeding schedule: small amounts 3-5 times a week minimum. For new or thin fish, I do smaller feeds more often.

How they behave and who they get along with

They are shy, mostly nocturnal, and pretty chill once settled. You will see a head poking out, then a quick dart back in. That is normal. The goal is a fish that comes out to grab food, not a fish that patrols the tank.

• Temperament: generally peaceful, but they will eat very small tankmates if it fits (tiny shrimp, tiny goby fry, etc.).
• Best tankmates: calm reef fish that do not harass the rockwork (small gobies, assessors, firefish in larger tanks, peaceful wrasses that are not constant hunters).
• Avoid: aggressive dottybacks, big hawkfish, large wrasses that pick at crevices, and anything that will outcompete it at feeding time.
• Inverts: larger cleaner shrimp are usually fine, but small sexy shrimp and tiny decorative shrimp are a gamble.

Breeding tips

Breeding in home aquariums is not common, mostly because people rarely keep pairs and you almost never witness courtship in the open. Like other brotulas/cusk-eels, they are secretive, and spawning would likely happen deep in a crevice.

• If you want a shot: start with two juveniles in a bigger tank with lots of separated hides so they can choose to be near each other without being forced.
• Keep the tank steady: stable salinity and temperature, no big swings, and plenty of food over time.
• Watch for pairing behavior: sharing the same crack system, not just tolerating each other across the tank.

Common problems to watch for

Most losses are not from disease first. They are from the fish never really settling, then slowly starving in the rocks.

• Not eating: the big one. Look for a pinched belly or a "paper thin" look behind the head. Target feed at the hide entrance and switch to smaller, smellier foods.
• Getting bullied: even "semi-peaceful" fish can ruin them if they hover and peck near the same cave. Provide multiple deep hides and be ready to move the bully, not the brotula.
• Jumping: use a tight lid and block gaps around cords.
• Mystery deaths after adding meds: they can be sensitive. If you have to treat, consider a separate hospital tank and keep oxygen high.
• Parasites/skin issues: because you do not see them much, do a real quarantine if you can. A fish that never comes out is hard to evaluate once it is in the reef.