Dubious conger

Where they come from

Dubious congers are deepwater eels from the western Atlantic and Caribbean, usually well off the reef on the continental slope. Think 100-300+ meters down where it is cold, dark, and quiet. They spend a lot of time buried in soft bottom, poking out to snag passing prey.

Setting up their tank

Give it room, darkness, and cold, well-oxygenated water. A 6-foot tank is a realistic minimum for an adult, and longer is better. They are burrowers, so plan the layout around that behavior.

• Chiller sized to hold 10-16 C (50-61 F) without big swings
• Deep soft sand bed (5-10 cm) with sections built up to 15 cm for burrowing
• PVC caves/tunnels (50-75 mm diameter) partially buried in sand
• Very tight lid with weighted or latched access panels
• Covered overflows and cable cutouts (mesh or foam gaskets)
• High aeration and surface agitation; oversized skimmer helps
• Moderate laminar flow along the bottom, not blasting the burrow
• Dim lighting - moonlight-level is plenty

Run full-strength seawater at 34-35 ppt, pH 7.9-8.2, with zero ammonia and nitrite, and low nitrate. Keep temperatures stable within that cold range and avoid rapid changes. These eels come from high-oxygen water, so run strong gas exchange and do not let CO2 build up in a closed room.

What to feed them

They are ambush predators that eat small fishes and crustaceans. In captivity, they take meaty marine foods offered on tongs once they settle. Start with movement to trigger a strike, then switch to steadier target feeding.

• Silversides or sand lance pieces (not as a sole diet)
• Squid strips and tentacles
• Mysis and chopped prawn/shrimp
• Clam, mussel, and scallop chunks
• Lean marine fish fillet (hake, cod, pollock) in small pieces

How they behave and who they get along with

Shy, mostly nocturnal, and very keyed in to vibration. You will see a head poking from the sand with quick strikes at dusk. They are not mean for the sake of it, but anything bite-sized is food. They ignore sturdy, non-nippy tankmates that do not invade their burrow.

• Best as a species-only display or with a few large, calm fish that handle 10-16 C and dim light
• Skip small fishes, shrimps, and crabs - they will disappear
• Avoid boisterous or nippy species that harass at the burrow
• Do not mix with other burrowing eels unless you have a very large footprint and multiple separated zones

Breeding tips

Not something you can do at home. Like other conger eels, they have a leptocephalus larval stage and spawn offshore. No documented captive breeding for this species, and even public aquariums have not cracked it. Focus on long-term holding and stable conditions instead.

Common problems to watch for

• Warm temps and low oxygen - leads to lethargy, poor feeding, and sudden deaths. Run a chiller and heavy aeration.
• Refusing food - keep lights low, offer at dusk, try moving prey on tongs, and rotate foods. Check temperature first.
• Escape attempts - seal every gap. Add weight or clips to lids. Cover overflows with rigid mesh.
• Nose and jaw abrasions - happen from rubbing on glass or netting. Use a container to move the eel, not a net, and give soft sand with rounded PVC edges.
• Barotrauma from collection - some arrive with gas issues, bloating, or trouble staying down. Many do not recover. Ask suppliers how the fish was decompressed and be ready to pass if the fish looks off.
• Infections from injuries - keep water pristine and treat in a dark, covered hospital tank if you see redness or fungus.
• Fatty liver from rich diets - do not power-feed daily. Use lean marine meats and keep a varied menu with vitamins.