Kobe flounder

Where they come from

Kobe flounder (Crossorhombus kobensis) is a right-eyed flounder from the western Pacific, most associated with Japan and nearby waters. They spend their lives on sandy or mixed sand-mud bottoms, tucked in and waiting for food to come to them. If you have only kept "reef" fish, this one feels like a different hobby.

Setting up their tank

This is an expert fish mostly because the setup is unforgiving. They need a mature marine system, steady parameters, and the right substrate. A flounder that can't bury or is constantly bothered will stop eating and waste away fast.

• Tank size: I would not bother under 75 gallons, and 120+ is where they start acting relaxed.
• Footprint matters more than height. Long and wide beats tall every time.
• Substrate: fine sand, deep enough to cover the body (2-3 inches is a good target). Avoid sharp aragonite rubble.
• Rockwork: keep it stable and leave open sand. They want a runway, not a maze.
• Flow: moderate. You want oxygen and clean water, but not a sandstorm.
• Lighting: they do not care, but bright light with no shaded areas can keep them stressed. Give them some dim zones.

Cover every pump intake. Seriously. They will plaster themselves against intakes and overflows, especially at night or if spooked, and it can go badly in minutes. I use sponge guards or a screened box over anything that pulls water.

Filtration-wise, think messy carnivore. Big skimmer, aggressive mechanical filtration you actually clean, and a plan for nitrates. I like running a filter sock or roller plus a skimmer, and I vacuum the sand surface lightly during water changes (not deep stirring, just the top layer where the gunk lands).

What to feed them

They are ambush predators. Most new imports only recognize moving meaty foods at first, and they can be stubborn about "dead" food. The goal is to get them eating reliably and then broaden the menu.

• Best starter foods: live ghost shrimp, live blackworms (rinsed well), small live mollies acclimated to marine (use carefully, quarantine), or live shore shrimp if you can source clean.
• Transition foods: thawed mysis, chopped raw shrimp, chopped clam, pieces of silverside, squid strips.
• Long-term staples: varied marine meaty mix (mysis + clam + shrimp), plus occasional whole prey items for enrichment (small fish or shrimp).

Target feeding makes life easier. I use long feeding tongs or a feeding stick and wiggle the food right in front of their mouth. Once they learn the routine, they will often "perk up" out of the sand when you approach.

How often? Smaller individuals do better with small meals most days. Adults can do larger meals 3-4 times a week. Watch the belly line. A flounder that is always pinched behind the head is not getting enough, and a flounder that looks like a stuffed pillow is going to foul your water fast.

How they behave and who they get along with

Most of the time they are calm, buried, and basically pretending to be part of the sand. Then food shows up and they are lightning. They do not "play nice" with anything they can fit in their mouth, and they are surprisingly good at judging what might fit.

• Good tankmates: larger, peaceful-ish fish that ignore the bottom (bigger tangs, larger wrasses that sleep in rocks not sand, bigger rabbitfish).
• Bad tankmates: small fish, gobies, blennies, small wrasses, ornamental shrimp, small crabs - assume they are food.
• Problem tankmates: aggressive feeders (triggers, large puffers) that will outcompete them or harass them, and anything that picks at eyes or fins.

They can be stressed by constant motion over their head. If you keep busy midwater fish, give the flounder a quieter end of the tank with open sand and fewer "drive-bys."

Breeding tips

Breeding Kobe flounder in home aquariums is basically a long shot. Flounders typically spawn pelagic eggs in the water column, and the larvae go through a metamorphosis where the eye migrates. That larval phase is specialized and not something most of us can support without serious live plankton culture and dedicated rearing systems.

Common problems to watch for

• Refusing food after purchase: very common. Start with live foods, keep the tank calm, and dim the lights for a few days.
• Mouth injuries: they strike hard and can scrape themselves on coarse substrate or rocks while lunging. Fine sand and clear feeding zones help.
• Getting sucked onto intakes/overflows: cover everything that pulls water.
• Skin and fin infections: often follow shipping stress or abrasions. Keep water clean and avoid rough substrate.
• Internal parasites: wild-caught flounders can come in thin even if they eat. If they eat well but do not gain weight, suspect parasites and plan treatment in a quarantine system.
• Ammonia/nitrite sensitivity: they are not forgiving of "new tank" swings, and heavy feeding can push a system over the edge fast.

Quarantine is worth the hassle with this species. A bare-bottom QT is awkward because they want to bury, so I use a tub of fine sand in a removable tray or a shallow dish. That way you can still siphon waste and observe them, but the fish can behave normally.

The biggest success factor I have seen is simple: calm tank, fine sand, and a feeding plan you can stick to. If you can get them eating confidently in the first couple weeks, you are most of the way there.