Saikai grenadier (rattail)

Where they come from

Saikai grenadiers (Ventrifossa saikaiensis) are deep-sea rattails. Think cold, dark slopes off Japan and nearby waters, living down where the pressure is high, the food shows up in bursts, and the current is steady but not ripping.

That background explains basically everything about keeping them: they do best in coldwater marine systems, low light, lots of dissolved oxygen, and calm, predictable surroundings.

Setting up their tank

The biggest hurdle is temperature and stability. Mine did best kept consistently cold, with minimal day-night swing. They are also surprisingly easy to spook, so you want a tank that stays quiet and feels secure.

• Tank size: bigger is your friend. Aim for at least 180 gallons, and longer footprints beat tall columns. They cruise and hover more than you would expect.
• Temperature: coldwater marine. Most people who succeed keep them in the low teens C (mid-50s F) ballpark and avoid warm spikes at all costs.
• Salinity: standard marine strength, stable. Deepwater fish do not love quick swings.
• Filtration: heavy biofiltration and aggressive gas exchange. Big skimmer, big sump, lots of turnover without blasting the fish.
• Flow: moderate, directional flow is fine, but avoid a sandstorm. Think "steady current" not "surge zone."
• Lighting: dim. Give them shaded areas and do not hammer them with bright reef lights.

For decor, I like a simple slope of sand with scattered rockwork that creates overhangs. These fish are tail-heavy and can scrape themselves if you pack the tank with sharp rubble. Smooth structures and wide lanes beat a busy rock wall.

What to feed them

They are meat eaters and opportunists. In my experience they settle in faster if you start with real, smelly foods and only later try to transition to cleaner options.

• Good starters: chopped squid, prawn, clam, mussel, and marine fish flesh (sparingly).
• Frozen works well if it is high quality and rinsed. These fish do not need cloudy, oily water.
• Sinking foods: they are not built to chase flakes at the surface. Use feeding tongs or drop food right in front of them.
• Variety matters. Rotate foods so they do not get stuck on one item.

Target feeding is your best friend. I use long tongs and feed near the bottom, slightly up-current so the scent trail hits them. Once they start taking confidently, you can reduce the "hand feeding" and let them pick pieces off the substrate.

How they behave and who they get along with

They are quiet, sit-and-hover predators. Not aggressive in the "pick fights" way, but anything small enough to fit in the mouth is food. They also stress easily if housed with hyper, nippy fish.

• Best tankmates: other coldwater, deepwater-style fish that are calm and not bitey, and that are too large to be eaten.
• Avoid: fin nippers, fast surface feeders that keep the tank in chaos, and anything that will outcompete them at feeding time.
• Inverts: hit or miss. Small shrimp and crabs tend to become snacks. Large, armored coldwater inverts sometimes work, but do not count on it.

They do a lot of their "real" activity at low light. If you keep the tank bright and busy, you will think you bought a fish that just hides. Give them shade, calm flow, and a feeding routine, and you will see a lot more natural behavior.

Breeding tips

Realistically, captive breeding in a hobby setting is not something I would plan on. Grenadiers are deep-sea spawners with cues we cannot easily replicate (pressure, seasonal changes at depth, and wide-ranging behavior).

If you ever keep a group and notice swelling, changed behavior, or eggs, treat it as a rare observation and focus on water stability and low stress rather than trying to force a spawn.

Common problems to watch for

Most issues with Saikai grenadiers come down to shipping stress, temperature mistakes, and feeding problems during the first month. If you get past that period with a fish that eats steadily and stays calm, you are doing a lot right.

• Refusing food: common after import. Offer strong-smelling marine foods, keep lights low, and feed small amounts more often until it locks in.
• Rapid breathing or hanging in high-flow corners: often points to oxygen issues, warm creep, or dirty water. Check temp, surface agitation, and ammonia immediately.
• Mouth or snout damage: can happen from frantic dashes into rock or glass. Reduce light, add more shade, and remove sharp decor.
• Skin issues and parasites: imports can arrive rough. Quarantine is strongly recommended, but choose treatments carefully since deepwater species can react badly to harsh meds.
• Bloat/constipation: from oversized meals or too much rich food. Feed smaller pieces and mix in leaner items like squid and clam.