Rikuzen flounder
Dexistes rikuzenius
The Rikuzen flounder features a flattened body, primarily brown with lighter mottling, and a distinctively elongated pectoral fin.
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About the Rikuzen flounder
Dexistes rikuzenius is a demersal right-eyed flounder (Pleuronectidae) from the NW Pacific (Japan/Korea), living on sandy/muddy bottoms at ~42–200 m and feeding on benthic invertebrates.
Also known as
Quick Facts
Size
22 cm
Temperament
Semi-aggressive
Difficulty
Expert
Min Tank Size
55 gallons
Lifespan
5-10 years
Origin
Northwest Pacific (Japan and Korea)
Diet
Carnivore - benthic invertebrates (shrimp, crabs, worms), meaty frozen foods
Water Parameters
8-19.6°C
7.8-8.4
8-12 dGH
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This species needs 8-19.6°C in a 55 gallon tank. Use our heater calculator to find the right wattage.
Calculate heater sizeCare Notes
- Give it a big, wide footprint tank (think 4 ft long or more) with a soft sand bed 2-4 in deep - crushed coral or sharp sand will wreck the belly and fins.
- Keep salinity stable at normal marine levels; this species is from cool-temperate waters and occurs at ~42–200 m depth.
- They are messy ambush predators, so run oversized filtration and strong aeration; high nitrate and low oxygen show up as lethargy and rapid breathing.
- Feed meaty foods that sink: strips of shrimp, squid, clam, and marine fish flesh; use tongs to target feed so faster fish do not steal everything.
- Avoid small tankmates (they will become food) and avoid nippy fish that peck at eyes and fins; calm, cool-water species that ignore the bottom are the safest bets.
- Cover intakes and powerheads - flounders like to park and can get pinned, and sand can chew up unprotected impellers.
- Watch for skin scrapes, fin rot, and cloudy eyes from rough substrate or bullying; this species does not shrug off injuries like hardier flatfish.
- Breeding at home is basically a long-shot without seasonal cooling and live foods for larvae; if you ever see spawning behavior, you will need a separate rearing setup ready the same day.
Compatibility
Good Tankmates
- Other coldwater flatfish like starry flounder or small right-eye flounders - similar vibe, they mostly ignore each other if the tank is big and you have multiple sand patches
- Tankmates too large to be swallowed and that will not nip fins/eyes (species-specific compatibility not well-documented for this fish).
- Greenlings (Hexagrammos spp.) - tough enough to not get bullied, active but usually not dedicated fin-nippers, and they dont look like food to a bigger flounder
- Pricklebacks and gunnels - eel-ish rock hiders that keep to the cracks, usually dont compete much with a buried ambush fish
- Bigger coldwater sculpins (the chunky, stay-on-the-rock types) - works when neither can fit the other in its mouth and you have lots of caves so nobody has to share a favorite spot
Avoid
- Avoid small baitfish and tiny gobies/blennies - if it fits in the flounders mouth it will eventually be a late-night snack, even if it seemed fine for weeks
- Avoid aggressive bottom bruisers like large sculpins that are territorial or crabby - they will sit on the same sand edge and you will see fin chewing and stress
- Avoid triggerfish and big wrasses - too nosy and too bitey, and they love picking at anything that sits still on the bottom (your flounder will get harassed)
Where they come from
Rikuzen flounder (Dexistes rikuzenius) is a coldwater Japanese coastal flounder. Think northern Japan, sandy bottoms, eelgrass edges, and a life spent glued to the substrate waiting for food to wander past.
That wild lifestyle matters in the aquarium: they want cool, clean, oxygen-rich water and a bottom they can settle into without getting beat up by flow or tankmates.
If your system is geared for tropical reef temps, this is not a "just drop it in" fish. Plan a coldwater marine setup first.
Setting up their tank
A flounder tank is all about footprint, not height. Give them room to sprawl and hunt. I would rather keep one in a long, wide tank than cram it into something tall with the same gallons.
- Tank size: big footprint. A single adult really wants something in the 4 ft (or longer) range so it can settle, reposition, and not feel pinned.
- Temperature: coldwater. If you cannot run a chiller and keep it stable year-round, pick a different species.
- Substrate: fine sand. Skip sharp crushed coral or chunky aragonite - it can scrape the belly and fin edges when they bury.
- Rockwork: keep it minimal and stable. A few low, secure structures are fine, but leave open sand flats.
- Flow: moderate overall, but create calmer zones on the bottom. They do not enjoy getting sandblasted while trying to rest.
- Filtration: oversized and mature. These are meaty-food fish, and the waste adds up fast.
Use a "feeding tile" or a small flat dish on the sand. It keeps messy food out of the substrate and makes cleanup way easier.
Lighting can be pretty chill. They do not need reef-level intensity, and bright lights can keep them tucked away. If you want them out more, run a softer photoperiod and provide a couple shaded areas.
Lids matter. Flounders are not famous jumpers like wrasses, but startled fish do weird things, and coldwater systems often have strong surface agitation. Cover gaps.
What to feed them
They are ambush predators. In practice that means they do best on a rotation of meaty marine foods, and you have to watch that they actually get the food before faster fish steal it.
- Staples: chopped shrimp, squid, clam, mussel, marine fish flesh (from clean sources), quality frozen carnivore blends
- Occasional: live foods to start new or shy individuals (ghost shrimp acclimated to marine, small shore crabs where legal, or live marine shrimp if you can source them safely)
- Avoid as a main diet: feeder goldfish/rosies (fatty and not marine), freshwater worms as a routine food, anything with questionable parasites
Most new ones need a little coaching to take frozen. I have the best luck using feeding tongs and gently wiggling a piece of shrimp right in front of the mouth. Once they connect "tongs = dinner," life gets easier.
Feed smaller portions more often at first. Big chunks can be taken, but you will foul the tank faster and you can miss early signs of refusal.
Do not leave uneaten seafood rotting in the sand. Pull it after a few minutes. The tank can look clean while the ammonia spike is already starting.
How they behave and who they get along with
Rikuzen flounders are calm most of the day, then suddenly very efficient at feeding time. They spend a lot of time buried with just the eyes showing, and they will pick a favorite spot and return to it.
The main rule with tankmates is simple: if it fits in the flounder's mouth, it is food. Also, anything that likes to nip fins or pester bottom fish will stress it out.
- Good tankmates: peaceful coldwater fish too large to swallow, non-nippy species, calm midwater fish that do not race for every bite
- Risky: small fish, shrimp, crabs (often become snacks), aggressive feeders that steal food, fin nippers
- Also risky: stinging anemones/corals in a mixed setup (this species is better in a fish-only coldwater system)
Do not keep it with tiny "cleanup" animals you are attached to. A flounder that has settled in and is eating well will eventually test anything that moves on the sand.
One more thing: they can be a little shy in a busy tank. If you want to actually see it, keep the stocking sensible and the feeding routine consistent.
Breeding tips
Breeding this species in home aquariums is basically a research-level project. In the wild they are seasonal spawners, and getting the temperature swings, photoperiod, conditioning, and then raising tiny marine larvae is the hard part.
If you ever get a pair and want to try anyway, focus on conditioning first: heavy (but clean) feeding, excellent oxygenation, and gradual seasonal changes rather than sudden parameter swings.
If you see surface activity and scattered eggs, you will need dedicated larval gear (greenwater, rotifers, and later Artemia) and a plan for metamorphosis. The larval stage is where most attempts crash.
Common problems to watch for
Most issues I see with flounders come from three things: warm water, dirty water from messy feeding, and damage from the wrong substrate or rough decor.
- Heat stress: rapid breathing, hiding constantly, refusal to eat. Fix the temperature problem, not the symptoms.
- Ammonia/nitrite spikes: common after big feedings. Test often, export waste, and do not overstock.
- Belly and fin abrasions: from coarse substrate or sharp rock. You will see redness or frayed edges.
- Food competition: flounder loses weight because faster fish steal meals. Target feed with tongs and use a feeding station.
- Parasites and shipping damage: wild fish can arrive scraped or with external parasites. Quarantine is your friend.
Watch the body shape, not just whether it eats once. A flounder can take a bite and still slowly waste away if it is being outcompeted or dealing with internal parasites.
If you keep the temp steady on the cool side, give it a clean sand flat, and run the system like you mean it (big skimmer, strong biofilter, regular water changes), they are hardy in their own way. They just do not forgive shortcuts.
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