Cartier's deepwater snake eel

Where they come from

Cartier's deepwater snake eel (Benthenchelys cartieri) is one of those animals that reminds you the ocean is basically another planet. They show up on deep slopes and muddy bottoms, well below the comfy reef zone most of our hobby fish come from.

That deepwater origin drives almost everything about keeping them: they are built for dim light, cooler and steadier water, and a life spent buried with just the head poking out waiting for food to drift by.

Setting up their tank

Give them space to disappear. A deep sand bed is not optional in my experience. Mine settled in the day I gave it 4-6 inches of fine sand and stopped trying to wedge under rocks.

Go for a larger footprint over height. You want a long tank with calm zones where the sand stays put. Flow that blasts the bottom turns their world into a sandstorm and you'll see them stressed and roaming.

• Tank size: I'd start at 125 gallons for an adult-sized eel, more if you want tankmates.
• Substrate: fine sand, 4-6 inches. Skip sharp crushed coral.
• Rockwork: stable and on the glass, not sitting on sand they can undermine.
• Lighting: low to moderate. They do not need bright reef lighting and often hate it.
• Flow: moderate overall, but keep the bottom calmer with smart pump placement.
• Filtration: heavy. Big skimmer, lots of biological capacity, and carbon helps with meaty-food funk.

Temperature is the make-or-break piece. This species is from deep water, so standard 78-80F reef temps are not where I'd keep them long term. A chiller and a stable controller setup is the realistic path here, and stability matters more than chasing a specific number. If you can't keep temps steady (especially in summer), I would not attempt this eel.

What to feed them

They are ambush predators. Once they learn your routine, they will take food from tongs, but most start off shy and will only eat when they feel hidden. Feeding at dusk with lights dimmed made a big difference for me.

• Best staples: thawed marine meaty foods like shrimp, squid, clam, scallop, marine fish flesh (sparingly).
• Also good: quality frozen carnivore blends and chopped raw seafood from a trusted source.
• Avoid: freshwater feeder fish, goldfish, and anything oily/dirty that fouls water fast.
• Portions: smaller pieces more often beats one huge meal.
• Schedule: juveniles 3-4 times/week, adults 2-3 times/week (watch body condition).

Keep an eye on the "neck" area behind the head. A healthy snake eel has some thickness there, not a pinched look. If it's getting skinny, bump feeding frequency and make sure tankmates are not outcompeting it.

How they behave and who they get along with

Most of the time you will see a head sticking out of the sand, watching the room. They're not a constant cruiser like some morays. At night they may roam more, especially if they feel underfed or exposed.

They are predators, but not usually bullies. The risk goes the other way: pushy fish can stress them out and keep them from eating. Calm, deep-bodied tankmates generally work better than zippy sand-sifters.

• Good fits: peaceful to semi-peaceful fish that won't harass the eel (bigger angels, tangs, some wrasses if not too hyper).
• Avoid: small fish you wouldn't mind disappearing, tiny gobies/blennies, and anything that sleeps in the sand.
• Avoid: aggressive triggers, large puffers, and fish that nip faces and fins.
• Avoid: crustaceans you care about. If it fits in the mouth, it is food.
• Bottom crew: choose snails and tougher inverts carefully, and accept losses.

Breeding tips

Breeding this species in home aquariums is basically not a thing. Deepwater eels have complex larval stages (leptocephalus larvae) that drift in the plankton, and getting from spawning to settlement is way beyond what most of us can pull off.

If you keep more than one, don't count on a "pair." You might just be housing two predators that tolerate each other until they don't. If you ever see serious biting or one eel hogging the only good burrow, separate them.

Common problems to watch for

The big ones I see with deepwater snake eels are stress from exposure, heat, and feeding issues. They can look "fine" right up until they stop eating and then they slide fast.

• Not eating: usually from too much light, too much bottom flow, harassment by tankmates, or a tank that is too new.
• Mouth/face injuries: from hitting lids, rock edges, or fighting tongs. Keep rockwork smooth near the burrow zone and use soft-tipped tongs.
• Skin damage and infections: often tied to sharp substrate, poor water quality, or repeated handling. Fine sand and stable water help a lot.
• Escapes: almost always a lid or overflow gap problem, especially after a water change or rearranging pumps.
• Rapid breathing/lethargy: check oxygen, temperature spikes, and ammonia immediately. Meaty feeding plus warm water can crater oxygen overnight.

If you want a single takeaway: treat this eel like a deepwater animal, not a reef eel. Dimmer, cooler, calmer, sandier, and very stable. If you can give it that, they are fascinating to keep. If you cannot, they tend to slowly fall apart and it's rough to watch.