You don't get the grill or stove warm enough
The pan or grill should be searing hot. When cold fish proteins come in contact with hot metal, they form chemical bonds that are extremely tough to break. It’s why you end up leaving part of the fish behind when you flip it. If you’re cooking your fish on the grill or stovetop, be patient. “Put your pan on the stove and turn the burner on medium heat for three to five minutes before placing your fish,” says Giuseppe Tentori, executive chef and partner at GT Fish & Oyster, a seafood restaurant in Chicago. When the pan or grates are extremely hot, the fis is easier to flip.
You cook it for too long
Cooking fish too long will dry it out and cause it to lose its natural flavors. A good rule of thumb: Measure the fish at its thickest point and cook for 10 minutes per inch, flipping halfway through. The flesh should feel firm and turn from translucent to opaque or white, but still be slightly translucent in the middle. “Take it off when there’s just that little bit of translucency left in the middle,” says Randy Hartnell, former Alaskan fisherman and founder of Vital Choice, a website that sells fresh seafood. “You can apply this tip to almost any type of fish. If it’s almost done, it’s done.”
For salmon, look for white lines to stand out. “When salmon is almost done, some of the proteins—which are a white color—will begin to leak out of the fish because they’ve coagulated,” says Tentori.
You touch it too much
Poking at your fish will cause its crunchy outer layer of skin, which traps delicious juices inside the fish, to fall apart as it cooks. Keep flipping to a minimum, and never use tongs. “Use one or two rubber or metal spatulas, or even a spoon, to flip it,” says Tentori. If fish is ready to be flipped, you will be able to easily slide a spatula underneath it. If you try prying it off too early, its crispy layer will stick to the pan.
You thaw incorrectly
Thawing at room temperature or with hot water provides ample opportunity for bacteria to grow. Instead, thaw fish for four to five hours in the refrigerator. If you need to thaw quickly, seal it in a zip-top bag and place it in a bowl filled with very cold water. “The bag should be completely covered with water, but make sure the meat itself doesn’t come into contact with the water,” says Tentori. “If it does, it will become too moist and very hard to cook.” This method will thaw a filet of fish in about 20 to 30 minutes.
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You over-salt the marinade
If you flavor marinade mostly with salt, the meat will absorb the liquid but leave most of the salt crystals behind. Rub the majority of seasonings and salt directly on the fillet, adding only a small portion of the salt to the marinade. Be careful not to marinate for too long, which will make the fish soggy. Mix dill, fennel, lemon, and a little bit of olive oil for a subtle yet delicious zest.
You buy the biggest lobster tails
You may want to impress dinner guests, but smaller lobster tails are sweeter and have a smoother texture. Cook tails in boiling water for about four minutes.
You don't examine the fish closely before buying
Start with the eyes. A whole fish should have a clear, bright eye (not a clouded, murky one). “The fish’s eye is the window into its age and the way it was treated,” says Hartnell. A whole fish should look shiny, and there should be no sliminess covering its surface (a sign of decay). Fresh fish flesh is firm and bounces back when you press into it. And, as counterintuitive as it may be, fresh fish should not smell “fishy.” It should smell like the ocean. A “fishy” smell means that fats inside the fish have begun to oxidize, a sign of decay and age.
You remove the skin before cooking
The skin will be easier to remove if you cook the fish skin-side down first. Cooking loosens the binding layer of fat between the meat and the skin, making it easy to peel off. The tough proteins in the fish skin also make it easier to flip and move around the pan. “Salmon must have the skin left on during cooking to crisp up nicely,” says Tentori. (However, other types of fish that are often thick cuts, like snapper, may take too long to cook if the skin is left on. Ask your fish monger for advice about whether to leave skin on or off if you're unsure).
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You overcook or undercook your shrimp.
Check the shape of your shrimp. Straight shrimp are undercooked, shrimp that have just curled into a C-shape are perfectly cooked, and shrimp that have twisted into an O-shape are overcooked. “Cook for about a minute and half on each side of the shrimp,” says Tentori.