Giant triangular batfish

Where they come from

Malthopsis gigas is a deep-slope sea batfish, not the tall, silver Platax you see in reef tanks. Think continental shelf and slope, on soft sand-mud bottoms, way down where light barely reaches. Most that show up in the hobby are bycatch from trawls. They are built for the seafloor: low, armored body, little lure on the snout, and they shuffle more than swim.

Setting up their tank

If you have not run a chilled, low-light, species-only marine system before, this fish will push you hard. Mine only settled once I treated the tank like a quiet, cold mudflat.

• Temperature: 8-14 C (46-57 F). Aim for 10-12 C and keep it steady. You need a reliable chiller sized for the whole system.
• Salinity: 35 ppt (1.025-1.026). pH 8.0-8.3. Ammonia and nitrite 0, nitrate under 20 ppm.
• Footprint over height: a 90-120 gallon tank with at least a 48 x 18 in footprint works for a single fish. They do not need height; they need floor space and stability.
• Flow: very gentle, diffuse flow. They hate being buffeted. Spray bars or low-output return split across multiple outlets works well.
• Lighting: very dim. Think dawn-level light. They get stressed under bright reef lighting.
• Substrate: 5-7 cm of fine sand mixed with silt. No sharp rock or coral skeletons. A few smooth, rounded stones are fine.
• Filtration: strong biofiltration with a skimmer. Cold water holds more oxygen but still run high aeration. Cover the tank to limit evaporation and temp swings.
• Mature system: give the tank months to grow microfauna. These fish do best in a stable, 'dirty-in-the-right-way' coldwater setup.

Quarantine matters. Deep-caught fish sometimes arrive with barotrauma or infections. I ran a 4-6 week chilled QT at the same temp as display. Keep it dim, bare-bottom with a sand pan, and lots of aeration.

What to feed them

They are ambush pickers. Mine would not chase anything. You have to bring the food to the mouth and let them do the little gulp.

• Starter foods: live saltwater mysids, small shore shrimp, and amphipods. Live blackworms can work in a pinch, but saltwater prey is better.
• Transition foods: thawed mysis, finely chopped shrimp, clam, scallop. Wiggle it on a feeding stick or baster right in front of the mouth.
• Feeding routine: pumps off, one-on-one target feeding. New arrivals eat daily in small portions; stable adults 3-4 times per week.
• Enrichment: soak thawed foods in vitamin mix and a bit of clam juice to trigger a response.

How they behave and who they get along with

Picture a quiet little rover that shuffles on its fins, parks, and waits. That is the vibe. They are not social, and they will ignore most things until something small gets too close.

• Best setup: species-only. They are slow, get outcompeted easily, and can be nipped.
• Tankmates to avoid: anything active or curious (wrasses, basslets), pickers (butterflies), cleaners, crabs that might pinch, and big stars that bulldoze.
• Inverts: small shrimp and crabs may become snacks. Snails are usually fine.
• Territory: they are not aggressive, but give them open sand to choose a spot.

Breeding tips

I would file this under not happening at home. Deep-slope batfishes likely spawn pelagic eggs and have a drifting larval stage. No documented hobbyist successes, and even public aquariums do not report closing the life cycle. Focus on keeping a single specimen stable and eating.

Common problems to watch for

• Refusing food: most common. Start with live prey, feed with pumps off, and be patient. Scented thawed food can work after a week or two.
• Heat creep: rooms warm up, chillers undershoot, temps rise. Use a controller with alarms and keep the chiller coils clean.
• Abrasions and sores: rough decor or nets scrape them. Treat early in QT with improved water quality and, if needed, a broad-spectrum antibiotic at coldwater-safe dosing.
• Barotrauma from collection: bulging eyes, trapped gas under skin, odd buoyancy. If it shows up on arrival, odds are poor. Gentle, dark QT and perfect water give the best chance.
• Copper and harsh meds: anglerfish relatives are sensitive. I avoid copper and formalin on these. Use targeted treatments (e.g., praziquantel for worms, nitrofurazone for bacterial issues) and lots of oxygen.
• Internal parasites: stringy white feces, weight loss. Medicated food with metronidazole or praziquantel helps if they are eating.
• Strong flow stress: frayed fins and refusal to move from corners. Dial the flow way down and diffuse it.