Cuban cusk-eel

Where they come from

Lucifuga subterranea is one of those fish that feels like it slipped out of a biology lab and into the hobby. They are Cuban cave fish (a cusk-eel relative) from underground systems where its dark, tight, and the food comes when it comes. That whole background is why they hate bright tanks, hate chaos, and do best in a setup that feels like a cave with water.

Setting up their tank

This is an expert fish because the tank has to be set up for the fish, not for you. Low light, lots of cover, and zero drama. If you can make the tank look a little boring at first glance, you are on the right track.

Go bigger than you think. A cramped footprint makes them bumpier, spookier, and harder to feed because a confident cave fish still wants room to glide and retreat. A long tank with a big bottom area beats a tall tank every time.

• Tank size: aim for a long footprint, not height (bigger is easier to keep stable)
• Lighting: dim or heavily shaded, with floating cover if needed
• Hardscape: rock piles, caves, and tight crevices they can wedge into
• Substrate: sand or fine gravel so they do not scrape up their underside
• Flow: gentle to moderate, no blasting current pointed at their hiding spots
• Filtration: strong biological filtration plus mechanical polishing (they like clean water)

For brackish, pick a target salinity and hold it steady. A lot of losses with oddball brackish fish come from people chasing numbers and swinging salinity around. Mix saltwater in a bucket, match temperature, and only then top off or do water changes.

What to feed them

These are not pellet pigs. Expect a picky, nocturnal feeder that does best on meaty foods. If you have kept predatory eels or burrowing fish, the vibe is similar: feed with intention, keep the water clean, and do not overdo it.

• Frozen: mysis, chopped shrimp, chopped clam, krill (sparingly), fish flesh from a trusted source
• Live (use carefully): ghost shrimp or small feeder shrimp can kickstart shy eaters
• Avoid as staples: fatty feeder fish, random bait shop stuff, and anything that makes the tank greasy

If yours will not eat in the open, feed after lights out and use a feeding stick or tongs. Once they learn the routine, they get a lot bolder. I have had the best luck offering smaller portions more often at first, then moving to a steady schedule.

How they behave and who they get along with

They are shy, secretive, and mostly active at night or in dim light. Once settled, they will cruise slowly and then vanish again. The biggest thing to understand is that anything small enough to fit in their mouth is on the menu, and anything too pushy will keep them pinned in a cave.

• Good tankmates: calm, midwater fish that ignore caves and do not outcompete at feeding time
• Bad tankmates: hyper fish, fin nippers, aggressive cichlids, and any fish small enough to be eaten
• Best setup: species-only or a very carefully chosen, low-stress community

They also do not love being jostled by fast feeders. If you keep tankmates, make sure you can still get food to the cusk-eel consistently. A hungry one will start taking bigger risks, and that is when you get injuries and escapes.

Breeding tips

Breeding this species in home aquariums is not something most hobbyists can count on. Cave fish reproduction can be tied to subtle seasonal cues (temperature shifts, food pulses, and changes in water chemistry) that are hard to replicate without stressing the fish.

If you ever want to try, your best shot is starting with a confirmed pair or group and giving them a mature, stable tank with lots of secluded spawning sites. Keep lighting low, keep feeding heavy but clean, and avoid big parameter swings.

Common problems to watch for

Most issues with Cuban cusk-eels are husbandry problems that show up late. They can look fine until they do not, so you want to watch behavior and feeding response more than color or flashiness.

• Refusing food: often stress from bright light, too much activity, or competition at feeding time
• Skin scrapes and infections: usually from sharp rockwork, rough substrate, or panic dashing
• Poor acclimation: quick changes in salinity or temperature can knock them back hard
• Nitrate creep: heavy meaty feeding plus weak maintenance leads to slow decline
• Escapes: loose lids, open cutouts, or gaps around plumbing

If you are troubleshooting, start with the basics: is it eating, is it getting food without competition, is the tank dim enough, and is the salinity stable week to week. Fix those first before you reach for treatments. Meds in brackish systems can be hit-or-miss, and stressed cave fish do not handle extra chemical stress well.