Cutthroat eel (Ilyophis robinsae)

Where they come from

Cutthroat eels (Ilyophis robinsae) are deep-water marine eels. They are the kind of fish that spend their lives in the dark, cruising over soft bottoms and ambushing small prey. If you are thinking reef tank or bright, busy community display, this is basically the opposite vibe.

Setting up their tank

Real talk: this is an expert-only animal because the tank needs to be built around its behavior. They want dim light, slack zones, and a place to disappear. If they feel exposed, they pace, stop eating, and bash their nose.

• Tank size: bigger is better for stability, but footprint matters more than height. Think long and wide so it can cruise and turn without scraping itself.
• Lid: 100% escape-proof. Any gap you can fit a finger in is a gap an eel will test.
• Lighting: low. Use shaded areas and let the tank have dark corners.
• Flow: moderate overall, but give it calmer areas behind rockwork or structures.
• Filtration: oversized and simple. These are messy carnivores, and you will likely be feeding chunky foods.

For the bottom, I lean toward fine sand. Deep-water eels often hang close to the substrate, and coarse gravel is just asking for abrasions. Rockwork should be stable and boring in the best way - no teetering towers. I like to build a few solid caves and then add lengths of smooth PVC or acrylic tube partly buried in sand so it has a snug retreat.

Temperature is another big thing people overlook with deep-water species. Many do better cooler than the average reef setup. If you cannot provide stable, species-appropriate temps (and keep them there), do not gamble on it. Stability matters more than chasing a specific number with swings.

What to feed them

They are predators. In captivity you are usually trying to get them onto non-living foods, and that can take patience. I have had the best luck feeding after lights-out with a long set of feeding tongs, keeping my hands out of the tank and letting the eel come to the scent.

• Scenty starters: pieces of shrimp, clam, squid, or marine fish flesh (use marine-sourced foods).
• Once eating: rotate meaty foods so it is not stuck on one item forever.
• Avoid: freshwater feeder fish and fatty freshwater meats - they can cause long-term issues.

Feed smaller portions more often at first, then space it out once it is steady. Watch the belly line. You want a healthy, filled-out look, not a pinched head and skinny body. And do not let food rot in the sandbed - it will wreck water quality fast.

How they behave and who they get along with

This is not a "pet eel that greets you" type. Expect a lot of hiding and nighttime activity. They can be surprisingly quick when they decide to move, and they do not do well with constant commotion from tankmates.

• Best setup: species-only or very quiet tankmates.
• Any fish small enough to fit in its mouth is food. That includes "it has been fine for months" fish.
• Avoid nippy fish. Fin and skin picking stresses eels out and leads to infections.
• Avoid aggressive bottom dwellers that compete for hides.

Also plan for the eel to rearrange things. Even if it does not dig like some eels, it will wedge into spots you did not expect. That is why stable rockwork and a good lid are non-negotiable.

Breeding tips

Breeding Ilyophis robinsae in home aquariums is basically not a thing. These are deep-water eels with a life cycle that likely involves pelagic larvae and environmental cues we cannot really replicate. If someone tells you they are "pairing them up" in a standard marine tank, be skeptical.

Common problems to watch for

Most issues come from stress and injuries, then water quality snowballs it. If the eel is not eating, that is your early warning sign. Do not wait weeks hoping it will "figure it out".

• Refusing food: often from too much light, too much traffic, or not enough hiding spots. Try dimming, adding a secure tube hide, and offering food at night with tongs.
• Nose and skin abrasions: caused by pacing, sharp rock, or trying to squeeze into rough holes. Smooth the habitat and give it a better hide than the one it keeps choosing.
• Rapid breathing or hanging in high-flow areas: can be oxygen or water quality related. Check ammonia, nitrite, and general gas exchange right away.
• Secondary infections: scrapes that turn red, fuzzy, or ulcerated need fast action and a quarantine plan.
• Escape attempts: usually a sign it feels exposed or the tank is too active, not just "because eels escape."

If you are determined to try this species, build the system like a quiet, dim predator tank first, then add the eel. Most failures I have seen come from trying to cram a deep-water hunter into a bright, busy display and hoping it adapts. It usually does not.