Always buy fish the day you want to cook it!!
Almost all fish, unless you bought it off the boat in your local port or harbor, came to where it is sold on ice or most likely frozen. It is then defrosted and sold. As it defrosts, it loses textural integrity and gets mushy: totally unappetizing.
Like cuts of meat, fish needs to rest and will continue to cook off the heat.
Most fish gets overcooked because cooks are afraid of it not being done in the center and overcook to compensate. You can stick a knife in the center of a thick steak (salmon, halibut, swordfish) to check its doneness. But it really will continue to cook as it rests.
Also similar to cuts of meat, the flesh becomes noticeably firmer once it is cooked.
All fish (and scallops and shrimp/prawns) will become more opaque as they cook, and the flesh becomes firmer. Notice how raw fish feels, because the flesh tightens up when it cooks, and often forms dense flakes of fish flesh.
Keeping these tips in mind will lead to beautifully cooked fish and happy diners! Add this super easy creamy garlic herb topping to any fish and they will be ecstatic!