Black mouth conger

Where they come from

Black mouth congers (Bathycongrus melanostomus) are deepwater marine eels. You are basically trying to keep a nocturnal burrower that evolved for dim light, cool stability, and ambush feeding. That explains 90% of the challenges people run into with them.

Setting up their tank

Think escape-proof predator tank with a burrow. These eels wedge themselves into holes and leverage their whole body to push. If there is a lid gap the width of your pinky, they will find it.

• Tank size: bigger is better, but plan around adult length and turning room. I would not even consider one in a small reef-style setup. A 180+ gallon footprint tank is a realistic starting point for long-term housing.
• Lid: tight-fitting, weighted or latched. Cover overflow teeth and plumbing gaps. Use mesh only if it is rigid and sealed all the way around.
• Rockwork: stable and sitting on the glass or on PVC supports, not on sand. They dig and can undermine piles fast.
• Burrows: PVC pipe sections (2-4 inch diameter depending on the eel) half-buried, plus caves with smooth entrances. Give at least two options so it can pick a favorite and feel secure.
• Substrate: sand is fine, but do not go super fine if your flow kicks up storms. A medium aragonite works well.
• Flow and oxygen: strong aeration and decent turnover. Big eels are messy, and oxygen drops at night can hit them hard.
• Filtration: oversize skimmer, lots of bio capacity, and a plan for nitrate control (water changes, refugium, or denitrification). They produce serious waste.

Lighting can be normal, but they act like a different animal under lower light or with a dusk period. If you want to actually see it out, use a dim blue viewing light after main lights go off.

What to feed them

They are carnivores and they eat like an ambush predator. New arrivals often refuse food for a bit, then suddenly decide everything smells edible. Patience helps, but so does offering the right stuff the right way.

• Best starter foods: fresh or frozen-thawed shrimp, squid strips, silversides, smelt, and marine fish flesh (not freshwater feeders).
• Use feeding tongs: you want your fingers nowhere near a hungry conger. Also, tongs let you wiggle the food and trigger the strike response.
• Feed at dusk or after lights out: you will get better response and less competition from bold daytime fish.
• Portion and schedule: 2-3 good meals per week is usually plenty for a settled eel. Smaller, more frequent feedings can help a skinny new fish gain back weight without blasting your nutrients.
• Vitamins: soak occasionally (especially if you rely on one or two food types). Variety is your friend here.

Watch the belly line. A healthy eel has some thickness behind the head and along the body. If it looks pinched or ropey, increase food variety and check for internal parasites.

How they behave and who they get along with

This is not a community fish. Expect a shy daytime eel that becomes bold and predatory once it is settled. Most losses in mixed tanks come down to size mismatch and nighttime hunting.

• Temperament: generally not interested in fish it cannot swallow, but opportunistic and persistent with anything it can.
• Tankmates that usually work: larger, sturdy fish that hold their own and are too big to be viewed as food (big angels, tangs, triggers with caution, larger wrasses).
• Tankmates to avoid: small fish, slender fish, bottom sleepers, and anything that likes to hide in the same holes (gobies, blennies, firefish, small wrasses).
• Inverts: assume shrimp and crabs are snacks sooner or later. Snails can get knocked around during digging.
• Other eels: risky unless the system is huge and you have multiple burrows. Even then, competition for dens can turn ugly.

One trick that helps: feed the eel on a consistent "spot" near its burrow. Over time it learns where food appears and you get fewer surprise lunges at passing tankmates.

Breeding tips

Realistically, captive breeding is not a thing for this species in home aquariums. Conger-type eels have complex life cycles with pelagic larvae, and mature adults may require environmental cues (depth, migration, temperature shifts) we cannot replicate in a glass box.

Common problems to watch for

Most issues with black mouth congers are husbandry problems that show up as "mysterious" behavior changes. If it stops eating or starts cruising the glass, something is usually off.

• Escape attempts: almost always a lid gap, a spooked eel, or poor shelter options. Add more burrows and seal every opening.
• Refusal to eat: common after shipping. Try feeding at night, offer shrimp/squid, and reduce daytime stress. Check for pinched belly and consider deworming with vet-guided meds if weight keeps dropping.
• Injury from rockwork: abrasions and bent jaws happen when they wedge into sharp rock. Smooth PVC dens prevent a lot of this.
• Poor water quality: heavy feeding plus big waste means ammonia and nitrite spikes can happen fast in under-filtered systems. Nitrate can climb quietly and then everything gets grumpy and sick.
• External parasites (marine ich/velvet): eels can show fewer visible spots because of their mucus. Watch for rapid breathing and flashing. Quarantine is hard with eels, but treating the display later is worse.
• Bacterial infections: red patches, cloudy eyes, or mouth damage after a feeding strike into rock. Clean water and early treatment matter.

If you only do two things, do this: give it a secure den it can fully hide in, and run filtration like you are keeping a messy predator (because you are). That combo solves a surprising number of headaches.