Windowpane flounder

Where they come from

Windowpane flounder are a cold-to-cool water flatfish from the western Atlantic, most common from the Gulf of St. Lawrence down past New England to the mid-Atlantic and into nearby coastal shallows. They like sandy or muddy bottoms in bays and nearshore waters, and their thin, almost see-through body is what gives them the "windowpane" name.

Setting up their tank

Give them floor space over water volume. A full-grown windowpane can hit 14-18 inches, so you want a long, wide tank. I would not even consider less than a 6-foot tank (180+ gallons), with as much open sand as you can manage. They live to bury.

• Temperature: 58-68 F (15-20 C). Plan on a chiller if your room creeps up.
• Salinity: 1.022-1.026
• pH: 8.0-8.3
• Substrate: 2-3 inches of fine, rounded aragonite sand. Skip sharp crushed coral.
• Flow: gentle across the bottom, strong gas exchange up top
• Lighting: dim to moderate. They relax in softer light.
• Filtration: oversized skimmer and a big biofilter. Meaty diets are messy.

Hardscape is optional. If you add rock, keep it stable and lifted on eggcrate so the fish cannot undermine it while digging. Leave big, open patches of sand.

Quarantine is worth the time. Bare-bottom QT is fine if you add a tray or container of fine sand so they can settle. They sulk on bare glass.

What to feed them

They are ambush predators. Wild ones eat small fish and crustaceans that wander too close. In the tank, many need a little convincing at first.

• Starter foods: live saltwater shrimp, live minnows caught from clean saltwater, or gut-loaded ghost shrimp if you must (short-term).
• Training foods: thawed silversides, sand eels, lancefish, squid strips, raw marine shrimp, and scallop. Wiggle the food on tongs right in front of the mouth.
• Routine: small meals once or twice a day at dusk works well. They are not sprinters; hold the piece until they inhale it.

You will know you are overdoing it if the fish balloons and stays exposed. Better to feed modestly and keep water quality tight.

How they behave and who they get along with

Most of the day they are sand-colored pancakes that barely move. Give them a quiet bottom and they will settle in. They are not aggressive in a chasing sense, but anything bite-sized that moseys by is food.

• Good tankmates: larger, calm midwater fish that mind their own business (big wrasses that do not pick, tangs, sturdy chromis groups, larger anthias in cool setups).
• Use caution: other flatfish. They can get grumpy over prime sand patches unless the tank is huge.
• Avoid: small gobies, blennies, shrimp, crabs, snails you care about, and anything that sleeps on the sand. Also avoid nippy or beaky fish like triggers and puffers that torment flatfish.

Breeding tips

Realistically, you are not breeding windowpane flounder at home. They spawn offshore with pelagic eggs and larvae that need specialized plankton foods and round tanks to keep them suspended. Even public aquariums treat this as a project. If you ever see courtship in a very large, chilled system, consider yourself lucky, take notes, and enjoy the moment.

Common problems to watch for

• Heat stress: panting gills, refusing to bury, blotchy color. Keep temps steady in the 60s and run strong aeration.
• Refusing food: new imports often need live prey to start. Try live saltwater shrimp at lights-down, then wean to thawed pieces.
• Skin and eye issues: abrasions from sharp substrate, or nips from tankmates. Use fine sand and peaceful company.
• Flukes and other parasites: very common on wild flatfish. Praziquantel works well during quarantine. Watch for flashing and excess mucus.
• Copper sensitivity: treat ich proactively via observation/QT and gentler methods. If you must use copper, dose carefully and monitor with a reliable test.
• Bacterial sores: can follow net damage. Move with a soft container, not a rough net, and keep the bottom clean.
• Nutritional gaps: a silversides-only diet leads to problems. Rotate foods and add a vitamin/HUFA soak.