Marshall's grenadier

Where they come from

Marshall's grenadier (Coryphaenoides marshalli) is a deep-sea rattail. Think cold, dark slopes and abyssal plains, way down where the pressure is wild and food shows up as occasional windfalls.

That background matters because almost everything we do in reefkeeping (bright lights, warm water, lots of daily activity) is the opposite of what this fish is built for.

Setting up their tank

If you are still determined (public aquarium-grade project, not a weekend experiment), the tank has to be built around cold, dark, calm water and a fish that spooks easily.

• Temperature: coldwater system, not tropical. You are in chiller territory.
• Lighting: dim. You want enough to observe without blasting it like a reef.
• Flow: steady but not blasting. Deep water is generally not a wave machine.
• Filtration: heavy duty, oversized bio and mechanical. These fish are messy on meaty foods.
• Oxygen: high. Cold water helps, but you still want strong gas exchange.
• Aquascape: open bottom with a few large, smooth structures. No sharp rock piles to scrape fins on. Give it a few shadowed areas to sit in.

Substrate-wise, I would go bare-bottom or a thin layer of fine sand. You want easy cleanup. Deep-sea fish plus chunky foods equals leftovers if you are not careful.

Feeding

In the wild they pick at whatever drifts down: carrion, crustaceans, worms, bits of fish. In captivity you are basically running a predator/scavenger feeding program.

• Start with: thawed marine meats like shrimp, squid, clam, scallop, and silversides-type fish (go easy on oily freshwater feeder stuff).
• Mix in: quality sinking carnivore pellets if you can get it to take them, but do not count on it.
• Offer variety: chunks and strips. Some individuals respond better to long, wiggly pieces (squid strips) than cubes.
• Feed schedule: smaller portions more often beats one huge dump. Deep-sea fish do not always handle "stuffed" feeding well in a tank.

If it arrives skinny (common), do not try to fix it with massive feedings. Get it eating reliably first, then slowly build it up.

Behavior and tankmates

They are not aggressive in the "chasing" way, but they are absolutely a mouth-first fish. Anything small enough to fit will eventually be treated like food, especially at night or in dim light.

• Best kept: species-only or with a couple of large, calm coldwater fish that will not compete like crazy at feeding time.
• Avoid: fast, nippy fish (they stress easily), and anything tiny or shrimp/crabs you care about.
• Activity: mostly hovering and slow cruising. If it is frantic or constantly crashing into things, something is off (light, water quality, too much flow, or just stress).

Breeding

I would not plan on breeding Coryphaenoides marshalli in captivity. They are deep-sea spawners, and we do not have a hobby-friendly playbook for triggering natural spawning cues (pressure, seasonal migrations, larval rearing at depth, etc.).

Common problems to watch for

Most of the problems are really "arrival and acclimation" problems. If it makes it through the first month eating and acting normal, your odds improve a lot.

• Barotrauma/decompression damage: buoyancy issues, bloating, inability to right itself. Often shows up right away.
• Refusal to feed: can be stress, too much light, too much activity around the tank, or just a rough import.
• Secondary infections: scrapes and pressure-related injuries can turn into bacterial issues fast.
• Nitrate and organics creep: heavy meaty feeding will push your system hard. Watch water quality like a hawk.
• Intake and corner injuries: they can wedge themselves or abrade fins/snout if startled.

If you are serious about keeping one long-term, treat it like a coldwater, low-light predator that hates surprises: stable temps, quiet room, careful feeding, and lots of observation. This is one of those fish where the setup and logistics matter more than any magic trick.