Marquesas dwarf flounder

Where they come from

The Marquesas dwarf flounder (Engyprosopon marquisense) is a tiny, sand-hugging flatfish from the tropical central Pacific around the Marquesas Islands. Think clear, warm reef areas with patches of sand and rubble where a fish can vanish by simply exhaling and wiggling under the substrate.

They are not a "display swimmer". Most of the time you will see a set of eyes and maybe the outline of a pancake in the sand, and that is exactly the point.

Setting up their tank

If you try to keep this fish like a normal reef fish, you will have a bad time. Set the tank up like a sand flat with some structure nearby, and you will suddenly find the flounder acts like it owns the place.

• Tank size: I would not do one in less than 20-30 gallons, and bigger is easier for stability. Footprint matters more than height.
• Substrate: fine sand, not crushed coral. I aim for 2-3 inches so they can bury fully.
• Rockwork: keep rock stable and sitting on the bottom glass or on supports, not perched on sand that can shift when the fish digs.
• Flow: moderate overall, but create a few calmer zones where the sand is not constantly blowing around.
• Lighting: they do not care. Your corals might. Give the flounder shaded spots and it will use them.
• Filtration: strong mechanical + a skimmer. These guys are messy eaters and uneaten food becomes a nitrate factory fast.

Cover every opening. A dwarf flounder can and will launch when spooked, especially at night. A tight lid and guarded overflow teeth save lives.

What to feed them

This is where most people fail. They are ambush predators that want moving meaty foods, delivered right to their face. If the fish is already eating frozen when you get it, you just won the lottery.

• Best starter foods: live enriched brine shrimp (as a transition tool), live ghost shrimp, small live mollies (acclimated to salt) if you must, and live blackworms if you can source them clean.
• Reliable long-term foods: thawed mysis, chopped shrimp, finely cut squid, and small pieces of marine fish flesh (not oily freshwater feeders).
• How to feed: use feeding tongs or a turkey baster and drop the food so it wiggles right in front of the eyes.
• Schedule: small meals daily at first. Once settled, many do fine with 4-5 feedings per week, but watch body thickness rather than a calendar.

If it ignores frozen, do not panic and dump more food in. Offer a couple live items to trigger the strike, then sneak in a piece of thawed mysis right after. I have had the best luck with that one-two punch.

How they behave and who they get along with

They are calm, cryptic, and mostly stationary. The flip side is that anything that fits in their mouth is food, and anything that bullies the sand bed will keep them stressed and hiding.

• Good tankmates: peaceful reef fish that stay mid-water (small fairy wrasses, small cardinals, some gobies that do not compete hard for the sand).
• Risky tankmates: aggressive feeders like dottybacks, large wrasses, hawkfish, triggers, big clowns, and anything that will steal every bite before the flounder reacts.
• Nope list: tiny ornamental shrimp and very small fish. If it can be swallowed, assume it will be.
• Other sand-sifters: avoid heavy sand-stirrers (many sea stars, big conchs, large nassarius mobs) that constantly run the flounder over or collapse its "burrow" all day.

You will sometimes see them "walk" along the sand with little fin ripples, then do a quick shiver to bury. If the fish is always on top of the sand and looks jumpy, something is bothering it - usually flow, tankmates, or a sand bed that is too coarse.

Breeding tips

Breeding this species in home tanks is basically a moonshot. Like other small flounders, they are broadcast spawners in the wild, and the larvae go through a long pelagic phase before metamorphosis (the classic eye migration happens later). That means you would be looking at a dedicated setup, live plankton cultures, and a lot of trial and error.

Common problems to watch for

• Starvation: the #1 issue. A new flounder that is not eating within a few days is on a timer. Look for a pinched body behind the head and a "thin pancake" look.
• Competition at feeding time: fast fish steal the food before the flounder commits to a strike. Target feed and consider distracting the other fish first.
• Sand-related injuries: belly abrasions, frayed fins, or red patches from rough substrate or getting pinned against rocks.
• Parasites and shipping stress: wild fish can arrive with flukes or protozoans. Heavy breathing, flashing, or cloudy eyes are red flags.
• Jumping: especially after lights out, during maintenance, or if chased.
• Nitrate creep: messy feeding plus a sand bed can push nutrients up if you do not export and siphon detritus.