Plainfin grenadier
Ventrifossa divergens
The Plainfin grenadier features a slender, elongated body with a pale, translucent coloration and a distinctive long dorsal fin.
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About the Plainfin grenadier
This is a deep-sea rattail from the Indo-West Pacific that cruises slopes 183-772 m down, so you will never see it in home aquariums. It reaches about 30 cm and has a plain dark first dorsal fin instead of the pale blotch a lot of its cousins show. Cool fish to read about, but it needs near-freezing water and deepwater conditions that we just cannot provide in tanks.
Quick Facts
Size
30 cm
Temperament
Semi-aggressive
Difficulty
Expert
Min Tank Size
0 gallons
Lifespan
unknown
Origin
Indo-West Pacific
Diet
Carnivore - benthic crustaceans, worms, small fishes; will scavenge
Water Parameters
6-12°C
7.5-8.1
30-40 dGH
Care Notes
- Run a big chilled system - think 500+ liters with a serious chiller holding 3-6 C, blackout the tank sides, keep lighting very dim, and use red light if you want to watch without spooking it.
- Give it floor space, not height: a 6-foot tank with a wide open runway, 2-3 cm of fine sand (no sharp rock), and gentle laminar flow along the bottom works best.
- Keep salinity at 35 ppt (SG 1.025-1.027), pH 7.9-8.2, ammonia/nitrite at 0, nitrate under 10 ppm, and crank oxygen with a strong skimmer or O2 reactor; cold systems cycle slow, so seed and mature the biofilter at the final temperature first.
- Feed small sinking pieces of marine seafood (clam, squid, shrimp, white fish) with tongs right in front of it, 3-4 times per week; skip oily fish, soak vitamins, and siphon leftovers immediately.
- House it alone or only with other deep-cold, slow, non-competitive fish; it will eat bite-size fish and shrimp, and fast feeders will outcompete it.
- Expect barotrauma from collection: keep lights off, handle minimally, and do not try to needle the swim bladder; if it struggles to stay down, park it in a soft-bottomed acclimation box on the substrate and give it time.
- Cover pump intakes and round off any hard decor - they spook easily and that long tail gets chewed up fast in grates and sharp rock.
- No captive breeding here - they are deepwater broadcast spawners - so focus on stable long-term holding and have backups for the chiller, pumps, and a low-temperature alarm because a temp spike can wipe it out in hours.
Compatibility
Good Tankmates
- Another grenadier of similar size in a huge, chilled, low-light setup - lots of floor space and 2-3 food drop spots keeps squabbles down
- Chill coldwater eelpouts (Zoarcidae) that stick to their holes and do not nip - too big to swallow, not pushy at feeding
- Big, slowgoing lumpfish/lumpsuckers that park on glass or rocks and ignore the bottom crew
- Large, non-predatory flatfish like big soles or sanddabs that mind their own patch and do not blitz the food
- Mellow midwater coldwater fish that cruise slowly and are too large to be a snack - the kind that do not dive-bomb the bottom at feeding
Avoid
- Anything nippy or aggressive - triggers, puffers, big wrasses, damsels - they will shred fins and steal every meal
- Fast, hyperactive pelagics and greedy feeders like jacks or mackerels - too pushy, the grenadier will hide and starve
- Predatory bottom bruisers - groupers, cod, lingcod, big sculpins - likely to bully or try to eat a rattail
- Bite-sized fish and crustaceans - a nocturnal grenadier will pick them off after lights out
Where they come from
Plainfin grenadiers are deep-slope rattails from the Indo-Pacific, cruising over silty and muddy bottoms on continental slopes in very cold, very dim water. Think a few hundred to well over a thousand meters down. They spend their time close to the bottom, following scent and tiny vibrations to find worms, crustaceans, and the odd bit of carrion.
This is not a home-aquarium fish. You are fighting temperature, pressure history, and stress from capture. Unless you have serious coldwater experience, a reliable chiller setup, and a plan for power outages, skip this species.
Setting up their tank
You are building a quiet, dark, cold slope in a box. Stability matters more than anything. Sudden light or temperature swings spook them, and they hit the glass hard when startled.
- Tank size and shape: 300+ gallons with a long footprint (6-8 ft) so they can cruise without turning constantly.
- Temperature: 4-8 C is their comfort zone; many will tolerate 8-10 C if acclimated slowly. Use an oversized titanium chiller and insulate lines and sump.
- Oxygen: Keep near saturation. Cold water holds oxygen well, but have strong gas exchange and backup air. Aim for calm, laminar flow across the bottom, not blasting currents.
- Salinity and pH: 34-35 ppt, pH 7.9-8.2. Keep it steady.
- Lighting: Keep it very dim. View under red light; they barely react to it.
- Substrate: 2-4 cm of fine sand with some silt. Smooth rock piles or PVC caves for line-of-sight breaks. No sharp edges for that long tail to catch.
- Filtration: Oversize everything. Nitrification crawls at low temps, so double or triple your bio media. Big skimmer, frequent small water changes, and pre-chilled top-off.
- Flow hardware: Guard every intake. They startle and can get pinned.
- Lid: Tight and weighted. Startle jumps happen.
Acclimate in the dark. Float to match temperature first, then a slow drip acclimation with lights off and pumps quiet. Give them a full day with the room dim before you even think about food.
What to feed them
They are opportunistic benthic feeders. Mine only got confident once I stopped trying to feed in midwater and let the food rest on the bottom like something they found while cruising.
- Starter foods: Live or very fresh options help. Live marine mysids, amphipods, small grass shrimp, and chopped marine worms (Nereis) work well.
- Meaty pieces: Thin strips of squid, clam, scallop, prawn, or small chunks of marine fish. Start small and soft.
- Frozen transition: PE mysis, chopped krill, and finely cut silverside. Soak in vitamins/HUFA once they are eating reliably.
- How to deliver: Use a feeding tube or long tongs and place food on the substrate. Turn off flow for 5-10 minutes. Feeding at dusk gets better responses.
- Frequency: 3-4 modest meals per week is usually enough at 6-8 C. Pull leftovers fast to protect water quality.
Bury tiny pieces in the sand at first. That nose-down foraging posture is your friend. Red light during feeding keeps them calm.
How they behave and who they get along with
Mostly slow and deliberate, then out of nowhere a short, explosive dash if startled. They patrol the bottom a few centimeters up, tasting and listening more than seeing. Once settled, they will ignore you until food shows up, and then you get that methodical hunt.
- Temperament: Shy, not aggressive, but will inhale bite-sized fish, shrimp, and crabs.
- Tankmates: Best kept alone. If you insist, only very calm, coldwater species that ignore food on the bottom. No fast swimmers, no nippers, no warmwater guests.
- Room setup: Keep the tank in a quiet, low-traffic space. Footsteps and door slams cause panic sprints.
Breeding tips
There are no home-aquarium breeding records for Ventrifossa divergens. Like other grenadiers, they are deepwater broadcast spawners with pelagic eggs and larvae. You would need seasonal cues, enormous space, and likely pressure changes to even attempt it. Treat every individual as a long-term display fish only.
Common problems to watch for
- Decompression damage from capture: Pop-eye, buoyancy oddities, gas bubbles under the skin, or delayed losses a week or two in. Only accept specimens collected and decompressed properly.
- Refusal to eat: Keep it dark and quiet, switch to live foods, feed at dusk, and place food on the bottom. Patience beats force.
- Abrasion injuries: Their long tail brushes decor during panic runs. Use smooth rock and rounded PVC, and dim the lights.
- Bacterial issues in cold water: Sores or reddening. Quarantine new fish. Avoid copper with scaleless or reduced-scale fishes; consult a vet or experienced coldwater keeper for antibiotic options.
- Chiller or power failures: Temperature creep and oxygen drop are lethal. Use a controller with alarms, battery-backed air, and a plan for outages.
- Biofilter stall: At low temps, ammonia control is slow. Oversize media, seed heavily, feed lightly, and test often, especially during the first months.
If you cannot hold 4-10 C rock steady and keep oxygen high in very low light, do not attempt this species. Most losses are husbandry environment and capture-related, not your day-to-day care.
Plainfin grenadiers are fascinating and very calm to watch in the right setup. They are also a huge responsibility. If you have any doubts, go with a more forgiving coldwater fish first to dial in your system.
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