Promethean goby
Varicus prometheus
The Promethean goby displays a slender body with a distinctive dark mottled pattern and vibrant blue markings along its fins.
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About the Promethean goby
Tiny deep-reef goby with a bright yellow body and chunky brown saddle markings that pop in photos. It was only just described in 2023 from 247 m off Roatan, so it is a true unicorn for hobbyists and basically never seen in the trade. If anyone ever tried it, think cold, dim, super-stable conditions more like a chilled deepwater setup than a warm reef tank.
Also known as
Quick Facts
Size
3.1 cm SL
Temperament
Peaceful
Difficulty
Expert
Min Tank Size
20 gallons
Lifespan
unknown
Origin
Western Caribbean
Diet
Carnivore - tiny benthic crustaceans and other micro-invertebrates
Water Parameters
18-22°C
8-8.3
300-400 dGH
Need a heater for this species?
This species needs 18-22°C in a 20 gallon tank. Use our heater calculator to find the right wattage.
Calculate heater sizeCare Notes
- Run a chiller and hold 68-72 F; mine hid and went off food when the tank crept past 74.
- Keep it dim with blue-leaning light and plenty of caves, overhangs, and tight crevices; think deep rubble wall, not open reef.
- Use gentle, oxygen-rich flow across the rockwork; it sulks in dead spots but also hates being blasted.
- Hold SG at 1.025-1.026, pH 8.0-8.3, nitrate under 10 ppm, and keep things steady; this fish crashes with swings.
- Start with live copepods or Calanus and finely chopped mysis, then mix in frozen; target-feed with a pipette right to its bolt-hole 2-3 times a day.
- Pair it with slow, polite fish like assessors and small cardinals; skip wrasses, dottybacks, hawkfish, and fast anthias that will outcompete it.
- Quarantine in low light with a tight lid, and once it is eating, run praziquantel for worms; most are wild-caught and they will jump when spooked.
- Pass on specimens showing floating, head-up posture, or bulging eyes at purchase; those are decompression hits and survival odds are lousy.
Compatibility
Good Tankmates
- Chill nano-reef gobies like neon gobies (Elacatinus) - same easygoing pace and they share perches without drama
- Clown gobies (Gobiodon) - perch-and-watch types that ignore a Varicus hiding in the rockwork
- Shy midwater fish like firefish and dartfish (Nemateleotris, Ptereleotris) - calm feeders that will not muscle it out
- Tiny, gentle wrasses like possum or pink-streaked (Wetmorella, Pseudocheilinops) - active but not bullies
- Peaceful cardinals (Banggai, pajama) - slow hoverers that leave bottom perchers alone
- Assessors (Assessor spp.) - cave-loving, mellow basslets that mind their own business
Avoid
- Dottybacks and pushy basslets (Pseudochromis, overly spicy royal grammas) - they harass shy gobies and steal all the food
- Sixline and similar pest-wrangler wrasses (Pseudocheilinus) - nonstop pickers that will hound a timid Varicus
- Damsels and territorial clownfish - too feisty and will claim the whole rock pile
- Predators or big perchers like hawkfish, larger wrasses, or small groupers - anything that can swallow or stalk a tiny goby
Where they come from
Promethean gobies (Varicus prometheus) are a deep-reef Caribbean species, found on dim, cool slopes and ledges well below typical scuba depths. Think twilight zone habitat with broken rubble, small caves, and a steady rain of plankton. That background explains a lot of their quirks in captivity.
They are rare in the hobby and usually collected deep. Expect a delicate fish that ships hard and needs a very settled tank.
Setting up their tank
This is not a nano goby for a bright reef cube. Mine settled best in a quiet, mature setup with low light and lots of micro-habitats.
- Tank size: 20-30 gallons for a single fish. Bigger is fine; small tanks swing too fast.
- Temperature: 72-76 F (22-24 C). They come from cooler water and perk up at the lower end of reef temps.
- Salinity: 1.025-1.026. pH 8.1-8.4. Keep it steady more than perfect.
- Light: dim to moderate. Provide shaded zones with overhangs. They dislike harsh, white light.
- Flow: gentle to moderate with eddies. They like to perch out of the direct blast but appreciate good oxygenation.
- Aquascape: fine sand plus several piles of small rubble, snail shells, and narrow crevices. Build a few tight caves they can fully back into.
- Maturity: run the tank for months and seed it with pods. A refugium helps a lot.
- Lid: tight-fitting. They are not chronic jumpers, but any spook can send them up.
Acclimate slowly, keep the room and tank dim, and cool the water a touch before release. I float, then drip for 45-60 minutes, and leave lights off the whole day.
What to feed them
Expect a reluctant feeder at first. Mine ignored most foods for a couple days, then switched on once it saw tiny moving items in the water column.
- Starter foods: live copepods (Tigriopus, Tisbe), live baby brine enriched with HUFA, and live mysids if you can get them.
- Frozen standbys: Calanus, Cyclops, baby brine (enriched), finely chopped mysis, PE Calanus, fish eggs (roe).
- Dry foods: only after they settle. I had limited luck with tiny soft pellets soaked in tank water and garlic. Offer sparingly.
- Feeding style: many short, small feedings beat one big dump. They pick at drifting items more than they chase.
- Enrichment: rotate foods and soak frozen in a quality vitamin/HUFA a few times a week.
Turn off the main pumps for 5-10 minutes and let food tumble in gentle circulation. They will scoot out from their hide when it is not blasting past.
How they behave and who they get along with
Quiet, cryptic, and very site-attached. They perch, watch, and make short dashes for food. Once they choose a nook, that is home.
- Temperament: peaceful, but they will defend a small patch from similar gobies.
- Tankmates that work: tiny peaceful fish that ignore the bottom, like small chromis, assessors, possum wrasses, and pipefish that are already eating well.
- Tankmates to avoid: hawkfish, dottybacks, larger wrasses, boisterous damsels, and anything that hunts small crustaceans. Cleaner shrimp are usually OK; big pistol shrimp can spook them.
- Mixing: best kept singly. A pair might work in a larger tank with many hides, but only if you are confident they are a matched pair.
They vanish into rockwork. Use mesh on overflows and cover any gaps around plumbing. I have found gobies in weirs more times than I want to admit.
Breeding tips
I have not seen captive breeding reports for this species. Like many gobies, they likely lay small batches of eggs in a cave and guard them, with tiny pelagic larvae after hatch. If you ever get a pair to spawn, you will need true microfoods (rotifers and copepod nauplii) and a dedicated larval setup. For most of us, this is a watch-and-appreciate fish, not a breeding project.
Common problems to watch for
- Post-shipping shock: deep-caught fish can suffer decompression and temperature stress. Keep them cool, dim, and low-stress the first week.
- Not eating: use live pods and enriched live baby brine to kickstart. Offer tiny amounts several times daily. Do not let food rot in their caves.
- Jumping and wedging: tight lids and rubble that cannot collapse. Secure rocks before adding them.
- Parasites: quarantine if you can. They are sensitive to rough handling and heavy meds. Gentle observation, optional deworming with praziquantel, and clean water go a long way.
- Bacterial issues: torn fins or scrapes from shipping can get ragged. Keep water pristine and flow steady for healing.
- Heat stress and low oxygen: they come from cooler, oxygen-rich water. Keep temps on the low side and maintain surface agitation.
Do not blast them with bright, new-fixture reef lighting. Ramp lights very slowly over days. A sudden noon-day sun look can pin them in hiding and kill appetite.
If you only do one thing: build a mature, pod-rich, low-light corner of your tank and let the fish settle on its own timeline. Once it feels invisible, it will start taking food you offer.
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