Flounder

Flounder keeps 1–3 days refrigerated and 6–8 months frozen, but in practice it is worth treating the refrigerator window as one to two days rather than three. Thin flatfish fillets degrade faster than anything else in the case.

Store flat, in a single layer, in a covered dish over ice. Stacking flounder fillets compresses the delicate flesh and traps moisture between them.

Flounder and sole fillets are often only a quarter-inch thick, which means an enormous surface area for their weight — and surface area is where bacteria live and where moisture escapes. The same geometry means they freeze and thaw very quickly, so refrigerator thawing takes well under an hour.

If you are freezing flounder yourself, interleave the fillets with parchment so you can separate them frozen, and wrap the stack tightly.

Flounder illustrates a general rule about seafood better than any other fish: surface area governs spoilage rate. A quarter-inch fillet has several times the exposed surface per ounce of a thick halibut steak, and every square inch of that is a site for bacterial growth and moisture loss.

The flesh itself is also unusually delicate. Flatfish have fine muscle fibers and sparse connective tissue — which is why flounder is prized for its tender, silky texture — and that same fragility means the structure collapses early. Flounder that is going over turns from firm and translucent to soft, opaque, and slightly mushy well before it develops a strong smell.

That sequence is the practical hazard. With a firmer fish, smell and texture degrade roughly in step. With flounder, texture leads, so a fillet can be noticeably past its best while still smelling acceptable.

The usual marine-fish chemistry applies as it ages — trimethylamine oxide converting to the sharp ammoniac trimethylamine — but by the time that is obvious, the fillet has been unpleasant for a while. Judge flounder by feel and by the calendar.