Jonathan's cusk-eel

Where they come from

Jonathan's cusk-eel (Neobythites jonathan) is one of those deepwater, tucked-away reef slope fish you almost never see in the average marine tank. They come from darker, deeper habitats where the light is low, the food shows up in bursts, and there are lots of little holes and overhangs to retreat into.

That deepwater background drives pretty much everything about how they act in captivity: shy by day, hunting by smell at night, and not interested in bright, busy reef life.

Setting up their tank

Think of this fish like a nervous, nocturnal ambush predator that wants a bunker. Give it a tank that is stable, quiet, and built around hideouts. I would not put one in a brand-new system. Let the tank run long enough that your parameters stop swinging and the micro-life settles in.

Tank size depends on the individual and what you keep with it, but you will regret going small. They use the whole footprint at night, and you want options for caves so it can pick a spot and relax.

• Go for a wider footprint over a tall display. More bottom area makes a big difference.
• Rockwork with real caves: not just arches, but deep holes they can fully back into.
• Soft sand or fine gravel. They spend time on and in the bottom and you do not want abrasions.
• Lower light zones or shaded areas. They tolerate light, they just do not love being blasted by it.
• Moderate flow with calm pockets behind rock. Let it choose where it wants to sit.
• A tight lid. Cusk-eels can surprise you with an escape attempt, especially during acclimation.

Water quality needs to be boring: steady salinity, steady temp, low nitrate, lots of oxygen. They are not forgiving of big swings. I run oversized skimming and strong surface agitation, and I keep a close eye on pH at night in systems with heavy rock.

What to feed them

Most of the time the challenge is not 'what do they eat' but 'will it eat where I can see it.' They are scent-driven and often feed after lights out. If you try to make them compete with bold fish at noon, you will think it is starving.

• Start with meaty, marine foods: chopped shrimp, clam, squid, scallop, and quality frozen carnivore blends.
• Enriched frozen mysis can work for smaller individuals, but I do better with chunkier items.
• Soak foods in a vitamin/HUFA supplement a couple times a week, especially early on.
• Use feeding tongs or a long pipette and place food right at the cave entrance.
• Feed after lights dim. A red flashlight helps you check that it actually ate.

Feeding schedule: smaller meals a few times a week works better than huge dumps of food. They will gorge if you let them, and then you get a messy tank and a fish with a stressed gut.

How they behave and who they get along with

They are shy, mostly nocturnal, and pretty stealthy. You will see a head peeking out of a crack, then later you will notice cleanup crew going missing. They are not 'mean' in the classic aggressive-fish way, but they are predators and will take what fits.

• Best tankmates: calm, non-competitive fish that will not harass it (think quiet deepwater-ish companions).
• Avoid: fast pigs at feeding time (big wrasses, aggressive damsels) that keep it pinned in its cave.
• Do not mix with small fish or tiny bottom dwellers you are attached to. If it fits, it is food.
• Shrimp and small crabs are a gamble. Some individuals ignore bigger cleaners, others do not.
• One per tank is the safe play unless you have a large system and a plan for territories.

A lot of success with this species is just respecting its vibe: dimmer zones, predictable routines, and no constant commotion around its hide. If you have to do major aquascape work, expect it to sulk for a while afterward.

Breeding tips

Breeding in home aquariums is basically a unicorn with this group. Even getting confirmed pairs is tough, and deepwater fish often have seasonal cues (temperature, day length, food pulses) that we do not naturally replicate.

If you ever did want to experiment, you would be looking at a dedicated, species-only setup with multiple hides, heavy feeding leading into a cooldown/warmup cycle, and a way to separate eggs/larvae. Cusk-eel larvae, like many marine larvae, would likely be tiny and demanding (live plankton, pristine water, round-the-clock attention).

Common problems to watch for

Most issues with Jonathan's cusk-eel come from stress, starvation-by-competition, and shipping damage rather than some weird mystery disease. Catch problems early and you will save yourself a lot of heartbreak.

• Refusing food: usually too much light, too much activity, or it is not finding food fast enough. Try night feeding and target feeding.
• Rapid breathing or hanging in high flow: check oxygen, ammonia, and pH. Deepwater fish can be touchy about low O2.
• Skin scrapes and infections: often from rough substrate or squeezing into sharp rock. Fine sand and smooth caves help a lot.
• Internal parasites: weight loss despite eating, stringy white feces. Quarantine and consider antiparasitic treatment with expert guidance.
• Barotrauma/shipping injury: odd buoyancy, trouble settling, lethargy from day one. Sometimes they recover, sometimes they do not.

My biggest tell that something is off is a fish that never relaxes into a chosen hide. Once settled, they usually pick a home base. If yours is constantly roaming in daylight, breathing hard, or wedging into the strangest spots, take that as a sign to check the basics: stability, oxygen, aggression from tankmates, and whether it is actually getting food.