Robust assfish
Bassozetus robustus
The Robust assfish has a streamlined body, featuring a prominent, elongated snout and a dark brown to olive-green coloration with pale spots.
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About the Robust assfish
Bassozetus robustus (robust assfish) is a deep-sea marine cusk-eel (Ophidiidae) with a circumglobal distribution in tropical to temperate waters. It occurs at great depths (reported to >1000 m and to over 4000 m). Reproduction is oviparous and has been described as producing buoyant/pelagic eggs (reported as occurring in a gelatinous egg mass in some references). It is not an aquarium species, as its habitat conditions (depth/pressure and deep-sea environment) cannot be replicated in typical home systems.
Also known as
Quick Facts
Size
64 cm SL
Temperament
Peaceful
Difficulty
Expert
Min Tank Size
100000 gallons
Lifespan
unknown
Origin
Circumglobal (tropical to temperate deep waters)
Diet
Unknown
Water Parameters
2-6°C
7-8.4
8-12 dGH
Need a heater for this species?
This species needs 2-6°C in a 100000 gallon tank. Use our heater calculator to find the right wattage.
Calculate heater sizeCare Notes
- This is a deep-sea cusk-eel type fish - give it a dim, quiet tank with lots of caves/overhangs and a big open sand/mud patch to loaf on. Bright reef lighting and constant foot traffic will keep it stressed and hidden.
- Not an aquarium species: this abyssal fish lives at great depth where pressure and other deep-sea conditions cannot be replicated in home aquaria. As such, standard temperature/oxygen "care" guidance is not evidence-based for typical hobby systems.
- Go for a large, wide-footprint tank because they sit and sprawl more than they swim, and they do not love blasting current in their face. Moderate flow plus sheltered low-flow zones works way better than a gyre-only setup.
- Feeding is meaty and simple: small chunks of marine fish, squid, shrimp, and other fresh/frozen seafood, preferably soaked in vitamins now and then. Target feed with tongs after lights-out so faster tankmates do not steal everything.
- Skip aggressive or hyper-competitive fish - triggers, big wrasses, and anything that will harass a slow bottom sitter will ruin it. Best tankmates are other coldwater, low-aggression species that will not pick at it or outcompete it at feeding time.
- Watch the belly and jaw: they will take big bites and can choke or regurgitate if you offer oversized pieces, so keep chunks modest. If it stops eating, check temp and dissolved oxygen first, then look for bullying and shipping damage.
- Breeding in home tanks is basically a non-starter - they are deepwater spawners and you are not going to replicate pressure/seasonal cues. If you ever see eggs, treat it like a lucky accident and focus on keeping water very clean because they will fungus fast.
Compatibility
Good Tankmates
- Peaceful eel-like or wormy bottom sitters that are not bitey and not huge (think mellow cusk-eels or similar deepwater burrowers) - they mostly just coexist and ignore each other
- Non-aggressive scavengers like small brittle stars and serpent stars - good cleanup crew and they do not hassle the fish
- Detritus crew like snails and smaller hermits - fine as long as the hermits are not the big, pushy kind that will climb all over resting fish
- Very peaceful shrimp that keep to themselves (peppermint-type vibes) - usually OK if the assfish is well-fed and the shrimp are not tiny snack-sized
- Slow, non-competitive fish that do not rush food (small, mellow deepwater damsel relatives or similar gentle pickers) - the key is nobody out-muscles them at feeding time
Avoid
- Typical aquarium tankmates (this is a deep-sea species not suitable for standard home aquaria)
- Anything aggressive or pushy - triggers, large wrasses, dottybacks, most hawkfish - they will harass a peaceful bottom lounger and stress it into the ground
- Nippy fin-biters and constant peckers (many damsels, some angels, pecking butterflyfish) - they pick at a resting fish and it never gets a break
- Big predatory mouths (groupers, snappers, lionfish, big scorpionfish) - if it can fit the assfish, it will eventually try, and even a failed attempt can wreck them
- Crustaceans that get grabby - big crabs, large mantis shrimp, and the meaner 'decorator' types - they will snag a slow bottom fish sooner or later
Where they come from
Robust assfish (Bassozetus robustus) are deep-sea cusk-eels. Think slope and abyss edges, cold, dark water, and high pressure. They are not reef fish that wandered into the hobby - they are purpose-built for the deep.
Real talk: this is an expert-only animal because deep-sea species do poorly with typical tropical-marine temps and bright, busy display tanks. Most losses come from trying to keep them like a normal marine fish.
Setting up their tank
If you are serious about this species, build the system around cold water and stability, not aesthetics. You are basically setting up a chilled marine predator tank with low light and a calm flow pattern.
- Temperature: cold. Aim roughly 39-50F (4-10C) depending on collection depth and what the specimen actually tolerates. A chiller is non-negotiable.
- Salinity: normal marine range (around 1.024-1.026), and keep it steady.
- Oxygen: pack it in. Cold water holds more O2, but these fish still appreciate big gas exchange and no stagnant corners.
- Lighting: dim. Give them a shaded zone and do not blast them with reef LEDs.
- Flow: gentle to moderate with dead spots eliminated. They are not built for surfing in a gyre.
- Aquascape: soft sand or fine rubble, plus caves and overhangs. Big PVC elbows hidden behind rock work also work great.
- Filtration: oversize it. These are meaty-food fish and the waste adds up fast.
I have had better luck using multiple small shelters rather than one big cave. They pick a favorite, but having options reduces stress when you need to clean or move something.
Tank size depends on the individual and how it behaves, but do not cram it into a nano. Give it room to stretch out and turn without scraping its face on rock. More water volume also buys you stability when you are feeding heavy.
Avoid rapid temp changes during water changes. Mix and chill new saltwater to match, and swap slowly. Cold marine systems punish impatience.
What to feed them
These are carnivores that expect animal-based foods. If you can get them eating reliably, you are halfway there. The other half is not fouling the water with leftovers.
- Good staples: thawed marine shrimp, chopped squid, strips of fish flesh from reputable sources, scallop, and quality frozen carnivore blends.
- Occasional treats: live or freshly killed marine crustaceans if you can source them safely (avoid introducing hitchhikers and pathogens).
- Feeding style: target feed with tongs or a feeding stick near their shelter. They often prefer food delivered to them rather than chasing it.
- Schedule: smaller meals 2-3 times a week usually beats daily dumping. Watch body condition and adjust.
Skip freshwater feeder fish and goldfish. Wrong fats, messy, and they drag in problems. Stick to marine-based foods.
If yours is shy, feed after lights down with minimal room traffic. I have also used a small dish on the sand for messy foods so you can pull leftovers easily.
How they behave and who they get along with
Expect a low-key, sit-and-wait predator vibe. A lot of the time they are tucked into a cave with just the head out, then they come alive when food hits the water.
- Temperament: generally not a brawler, but anything bite-sized is food.
- Tankmates: choose cold-water species with similar temp needs and a calm nature.
- Avoid: fast, nippy fish and anything that will harass it in its shelter. Also avoid small fish and shrimp you want to keep.
- Best setup: species tank or a quiet cold-water community with roomy hiding places.
A lot of compatibility problems are really temperature problems. Most common marine fish will not enjoy the cold water this fish wants, so plan tankmates around the chiller, not the other way around.
Breeding tips
Breeding Bassozetus robustus in home aquaria is basically in the "not happening" category. Deep-sea fishes often have spawning triggers tied to pressure, seasonal cues, and huge ranges that we cannot recreate. If you ever see courtship behavior, log it and share it, because that would be genuinely useful info.
Common problems to watch for
- Heat stress: the big one. Even "cool" reef temps can be too warm long-term. Watch for heavy breathing, lethargy, and refusal to feed.
- Shipping/collection damage: deep-sea fish can come in rough. Look for jaw issues, buoyancy weirdness, abrasions, and cloudy eyes.
- Ammonia spikes from meaty feeding: easy to do in a new or undersized system. Test often, especially early on.
- Bacterial infections from injuries: these fish like to wedge into tight spots and can scrape themselves. Keep shelters smooth and openings roomy.
- Starvation by shyness: they can lose weight if tankmates steal food or if feeding happens in bright light.
If the fish is not eating, do not just keep adding food. Fix the reason first: too warm, too bright, too much traffic, or too much competition. Then offer a strong-smelling thawed food (squid or shrimp) on tongs right at the cave entrance.
Copper and harsh meds can be risky with oddball deep-sea species. If you have to treat, do it in a dedicated hospital tank, go slow, and prioritize pristine water and stable cold temperature.
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