Deep-water arrowtooth eel

Where they come from

Dysomma intermedium is one of those deep-water, mud-and-rubble eels that most divers never see. They turn up as bycatch sometimes, usually from deeper slopes where its dim, cool, and there are lots of cracks to vanish into. That deep-water background explains almost every headache people run into with them in home tanks: light, temperature, and stress.

Setting up their tank

Think "secure bunker" first, display tank second. You want a system that stays stable, runs on the cooler side for marine, and gives the eel a place to disappear without scraping its face.

• Tank size: I would not bother under 75 gallons, and 90-120 makes life easier once it starts moving around at night.
• Temperature: aim cool (roughly 70-74F). Warm reef temps tend to push stress and fast metabolism in deep-water fish.
• Salinity: standard marine (1.024-1.026). Keep it steady.
• Flow: moderate. Enough to keep oxygen high, not so much that the eel gets blasted out of its hide.
• Lighting: subdued. If you run bright lights, give it shaded caves and overhangs.

The big trick is structure. I like building a rock "maze" with lots of tight slots, then backing that up with a sand bed it can nose into. PVC works too, and honestly it is sometimes safer than jagged live rock for the first few weeks.

Quarantine is also not optional at this difficulty level. Deep-water fish often arrive beat up and stressed, and a full display with aggressive tankmates is a rough place to recover.

What to feed them

Plan on target feeding. Mine would not compete in the water column, and it definitely would not chase pellets around with tangs and wrasses doing laps. Offer food after lights out or at dusk, and use tongs or a feeding stick so the eel learns the routine.

• Best staples: thawed marine meaty foods (silversides in appropriate size, chunks of squid, shrimp, scallop, clam, smelt, and other marine fish flesh).
• Good variety: enriched mysis or chopped krill for smaller individuals, but meaty chunks usually get the strongest response.
• Avoid: freshwater feeder fish, goldfish, and other fatty freshwater stuff - it can cause long-term issues.
• Supplements: soak occasionally in a vitamin/HUFA product if it will take it.

Feed smaller meals more often at first (every 2-3 days), then you can usually settle into a steady schedule. Watch the body shape. A healthy eel looks rounded behind the head and along the body, not pinched.

How they behave and who they get along with

Expect a nocturnal, secretive fish. Daytime sightings might be just a snout and eyes poking out. At night, it will cruise low, check holes, and try to ambush anything it thinks it can swallow.

• Temperament: not a "mean" eel, but it is a predator. If it fits in the mouth, it is food.
• Best tankmates: larger, calm fish that do not pester hides (bigger angels, tangs, larger cardinals, some triggers with caution).
• Bad tankmates: tiny fish, small gobies/blennies, ornamental shrimp, and anything that likes to pick at faces and fins (some wrasses and aggressive damsels can be a problem).
• Inverts: snails and most crabs usually get ignored, but shrimp are a gamble and often become dinner.

One more real-world note: deep-water species can be weirdly sensitive to "busy" tanks. Constant motion, bright light, and fish that shove their way into caves will keep it hiding and skipping meals.

Breeding tips

Breeding in home aquariums is basically a non-starter. Like many eels, they have a larval stage that drifts in the open ocean (leptocephalus larvae), and we do not have a reliable way to close that cycle at home. If you ever see two individuals paired up, consider it a fun observation, not a breeding project.

Common problems to watch for

Most losses with this species come from shipping stress, refusal to feed, and injuries from poor hides or bad tankmate choices. If you keep it calm, cool, and well-fed, you have a real shot. If you treat it like a standard reef eel, it usually goes downhill.

• Refusing food: common early on. Reduce light, offer tighter cover, feed at night, and try different scents (clam usually works).
• Mouth and snout abrasions: from sharp rock or repeatedly ramming into rough hides. Smooth out the den, add PVC, and keep water clean.
• Parasites (marine ich/velvet): deep-water fish can crash fast. Quarantine and be ready with a treatment plan that matches your system.
• Bacterial infections: often show up after injuries. Look for redness, swelling, cloudy patches, or rapid breathing.
• Escapes: almost always a lid/gap issue.