Some fish sold as fresh at the counter has actually been frozen and thawed. This is a product that may legally be sold, but it must be identified on the label, and the distinction matters to the buyer: thawed fish cannot be refrozen at home and has a much shorter shelf life. To tell them apart, look at the flesh, eyes, and water at the counter. If in doubt, buy only enough fish to cook that day.
The most reliable indicator is the texture of the flesh. In fish that has been frozen and thawed, ice crystals damage the muscle fibers from within; pressing the flesh leaves an indentation, and the texture does not spring back. The cut surface looks loose and ragged, and is prone to flaking apart. With fresh fish, the pressed area springs back at once and the cut surface feels firm. If buying fillets, check whether the edges are breaking apart; thawed fillets have frayed edges. The skin offers a clue, too: fresh fish has shiny, slippery skin, while thawed fish looks dull and its scales come off easily.
With whole fish, the eyes and gills offer secondary clues. A fresh fish has clear, convex eyes; in thawed fish, the eyes are often sunken and cloudy, and sometimes look bloodshot. Fresh fish has bright red gills, while thawed fish has dull gills with a brownish cast. These signs are not conclusive on their own, since fresh fish that has been sitting for a long time can change in the same ways; assess all three together, and treat texture as the most important indicator.
The counter itself offers clues, too. Liquid collected under thawed fish is cloudy and milky because fluid has leaked out of the cells; liquid left by fresh fish is clear and scant. Check whether the fish is sitting on ice or in meltwater. Also, batches of thawed products tend to look very uniform: the fish are all the same size and arranged unusually neatly because they came from a package. Consider the smell of the whole counter, too; a good fish counter smells mildly of the sea, not strongly.
The distinction matters most in the kitchen. Thawed fish cannot be refrozen at home; if frozen again, its texture is damaged again, it falls apart when cooked, and the safe storage clock does not start over. Cook this product the day you buy it. Its shelf life is shorter than that of fresh fish because the clock starts at thawing, and the buyer does not know when that happened. For that reason, do not leave thawed fish until the next day. If marinating this fish, shorten the time further because the already-softened flesh breaks down quickly in acid.
The simplest approach is to ask; the seller is required to give an accurate answer, and packaged products already have this information on the label. Ask directly: Is this fish fresh or thawed? If the answer is unclear, do not buy it, or buy only enough to cook that day. Thawed fish is not a bad product; it is simply a different product and should be used accordingly: cook it the same day, and favor it for steaming and dishes with plenty of liquid. There is no need to ask the same question in the frozen-food aisle, since the information is already on the package.