Precarius dragonfish
Eustomias precarius
The Precarius dragonfish features a long, slender body with bioluminescent organs and dark brown to black coloration, aiding in deep-sea camouflage.
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About the Precarius dragonfish
Eustomias precarius is a deep-sea dragonfish from the open ocean off the western central Atlantic (Caribbean area). Its whole deal is the classic dragonfish vibe - bioluminescent light organs and a mouth built for grabbing whatever drifts by in the dark. This is strictly a wild, deepwater animal, not something that can be kept alive long-term in a normal aquarium.
Also known as
Quick Facts
Size
unknown
Temperament
Aggressive
Difficulty
Expert
Min Tank Size
0 gallons
Lifespan
unknown
Origin
Western Central Atlantic (Caribbean - off Puerto Rico)
Diet
Carnivore - deep-sea predator on small fishes and crustaceans
Care Notes
- This is a deep-sea dragonfish, so skip the bright reef look - go dim, black background, lots of caves/PVC, and keep room lights low or it will just stay stressed and invisible.
- Cold, stable water is the whole game: aim around 50-59F (10-15C), salinity 1.024-1.026, pH 8.0-8.3, and keep nitrate as close to 0 as you can because they crash fast in dirty water.
- You will need a real chiller and tight lids - they spook-jump, and a temperature swing of a couple degrees can be enough to knock them off feeding for days.
- Feeding is the make-or-break: start with live foods (ghost shrimp, small marine fish, live mysis) and slowly wean to thawed silversides/krill/mysis on feeding tongs right in front of its face.
- Feed small portions 3-5 times a week, not big dumps - they gorge, regurgitate, and foul the tank, and they do way better when you keep the water clean between meals.
- Tankmates: basically none unless you are doing a species tank - anything that fits in its mouth is food, and anything bold will outcompete it and keep it hiding.
- Watch for mouth and snout damage from slamming glass during night freak-outs; dim the tank, block reflections, and avoid sharp rockwork so it does not shred itself.
Compatibility
Good Tankmates
- Big, tough deepwater scorpionfish and similar sit-and-wait predators (think: hefty Scorpaenidae types) - they do not care about the dragonfish's attitude, and they are not easy to swallow.
- Solid-bodied deepwater groupers/sea bass types (Serranidae) that are too large to fit in its mouth - they tend to hold their ground and ignore the light show.
- Adult moray eels (Gymnothorax spp.) in a big system - they cruise their own lanes, and the dragonfish usually learns fast not to pick that fight.
- Large, armored bottom guys like big toadfish/frogfish relatives with a chunky build - not fast, but not snack-sized either, and they are not easily bullied.
- Other robust deepwater predators of similar size and feeding speed (kept well-fed) - if everybody is built like a bruiser and you feed heavy, it can be workable.
Avoid
- Same-species or other dragonfish/loosejaw types - they tend to turn it into a bitey contest, especially at feeding time.
- Small reef fish like gobies, blennies, chromis, and juvenile clownfish - if it fits in the mouth, it is food, full stop.
- Slow, fancy-finned fish (lionfish with long streamers, longfin butterflies, etc.) - the dragonfish will harass or bite, and shredded fins happen fast.
- Nippy aggressive stuff like big damsels, triggers, and puffers - they will pick at it, and that back-and-forth escalation usually ends badly.
Where they come from
Precarius dragonfish (Eustomias precarius) is a deep-sea stomiid from the open ocean. Think mesopelagic to bathypelagic water - cold, dark, and food that shows up in short, unpredictable windows. The whole body plan makes sense once you remember it lives where almost everything is either trying to disappear or eat you.
Real talk: this is one of those fish that looks amazing in photos but is a brutal candidate for home aquariums. Even very experienced marine keepers usually lose deep-sea dragonfish to collection/transport stress or feeding failure.
Setting up their tank
If you are still set on trying, design the system around three things: cold water, low light, and zero chaos. Bright reef lighting and a busy community tank is basically the worst possible match.
- Tank size: I would not attempt under 180-300 gallons. Not because they are huge, but because stability buys you time when the fish refuses food.
- Temperature: cold. You are in chiller territory, not a typical tropical marine setup. Aim for deep-water temps, not 75-78F.
- Lighting: dim. Ambient room light or very subdued blue is plenty. Give it lots of shadow.
- Flow: gentle to moderate, no blasting powerheads. These fish are built for hovering and short strikes, not surfing a gyre.
- Hiding: overhangs and caves help, but open midwater is where it will often hang. Give it both.
- Filtration: oversized and quiet. Strong mechanical + skimming, but avoid microbubbles and sudden blasting from outlets.
- Cover: tight lid. They can startle and rocket upward, especially after shipping.
I like using red light for quick checks. You can observe without turning the fish into a panicked missile.
Acclimation is where a lot of attempts fall apart. Go slow, keep the room calm, and avoid chasing it around with nets. If you can transfer it submerged (bag to tub to tank) you will save slime coat and stress.
Pressure change is the elephant in the room with deep-sea species. Many individuals simply are not viable long-term after collection, no matter how good your husbandry is.
What to feed them
Feeding is the make-or-break point. In my experience with deep-sea predators, you win by getting them eating quickly, then widening the menu. Waiting for them to magically accept frozen on day one usually ends in a slow starvation.
- Best starter foods: live marine fish or shrimp of appropriate size (think small, slender prey).
- Next step: freshly killed offerings on a feeding stick, moved like live prey.
- Long-term goal: frozen/thawed marine items (silversides, smelt, lancefish, prawn pieces) IF the fish will take them.
- Supplements: soak thawed foods in a vitamin/HUFA product once it is reliably eating.
Use a dim tank and feed at the same spot. I have had much better luck training reluctant predators by making feeding time predictable and low-stress.
Do not overdo it with huge meals. These fish are designed for infrequent big scores, but in a tank a giant item can lead to regurgitation or injury. I would rather see smaller prey more often while it is settling in.
Avoid freshwater feeders (goldfish/rosies). They are a nutrition trap and can cause long-term fatty issues in marine predators.
How they behave and who they get along with
They are sit-and-wait hunters with a big mouth and a hair-trigger strike. Mine spent long stretches hovering or resting, then suddenly turned into a lightning bolt if something looked edible.
Tankmates are mostly a bad idea. Anything that fits in the mouth is food. Anything too big to eat will often stress the dragonfish, steal its food, or outcompete it at feeding time.
- Best option: species-only, or a dedicated cold-water predator system with extremely carefully chosen companions.
- Avoid: fast, aggressive feeders (most wrasses, triggers), nippy fish, and anything that will harass it in dim light.
- Also avoid: inverts you care about. If it can grab it, it will try.
They are not "mean" fish, just built around eating. Plan around mouth size, not personality.
Breeding tips
Breeding Eustomias in captivity is basically not a realistic home-aquarium goal. Deep-sea life cycles are still poorly understood for many species, and even keeping a single specimen stable and feeding is the hard part.
If you ever see two together, do not assume they are a pair. Some deep-sea fish have dramatic sexual dimorphism, and many arrive in rough condition where "pairing" attempts just add stress.
Common problems to watch for
Most issues trace back to stress and not eating. You are often dealing with a fish that has already had a rough chain of events before it ever hits your tank.
- Refusing food: the big one. If it is not eating within the first couple weeks, your odds drop fast.
- Shipping damage: torn fins, jaw injury, scraped flanks. These can get infected quickly in a stressed predator.
- Regurgitation: usually from prey too large, too frequent feeding, or being spooked right after a meal.
- Secondary infections: cloudy eyes, mouth rot, red sores. Quarantine is tough with deep-sea fish, but treating in a calm hospital setup can be the difference.
- Barotrauma-related issues: buoyancy problems, odd swimming, inability to settle. Sometimes they improve, sometimes they do not.
Keep a log for the first month: water temp, salinity, nitrate, feeding attempts, what it struck at, and when. Patterns show up fast, and you will make better decisions than you will from memory at 1 a.m.
If you are trying this species, set your expectations honestly. The wins are rare, and the fish does not forgive rushed changes. Stability, dim light, cold water, and a feeding plan you can execute every day - that is what gives you any shot.
Similar Species
Other marine aggressive species you might be interested in.
Banded stargazer
Kathetostoma binigrasella
This is a New Zealand stargazer that lives half-buried in sand or mud with its eyes pointed up, waiting to rocket upward and nail passing prey. It has those neat dark saddle-bands across the back (especially as a juvenile), and like other stargazers it is venomous with spines near the gill cover/pectoral area - definitely a look-dont-touch fish.
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Eustomias decoratus is a deep-sea dragonfish (family Stomiidae) from the western central Atlantic around Bermuda. Like other Eustomias, it is a pelagic predator built for the dark - long body, big mouth, and a chin barbel used in hunting and signaling. This is absolutely not an aquarium species in any normal sense, since its real habitat is open ocean at depth and it will not tolerate typical captive conditions.
Gladiator dragonfish
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This is a deep-sea barbeled dragonfish - long, jet-dark, and built like a little ambush predator with a huge toothy mouth. It lives way down in the bathypelagic zone and uses a chin barbel as a lure, so its whole vibe is "lights-out hunter" rather than anything you'd ever keep in an aquarium.
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This is hoki (also sold as blue grenadier) — a deepwater, slope-associated marine fish found around New Zealand and southern Australia (and also off South America). It reaches about 1.2–1.3 m and lives in deep, cool waters, making it unsuitable for home aquaria.
Johnston Island damsel
Plectroglyphidodon johnstonianus
This is one of those tough little reef damsels that acts like it owns the whole rock pile, especially once it settles in. Maxes out around 14 cm and will absolutely defend a favorite cave or coral head, but the blue eye and chunky "wide bar" look make it a really cool fish if you plan the tank around its attitude.
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