Rule of thumb on cooking fish: 10 - 12 minutes per inch of thickness. Just a rule of thumb. It always depends on the method and temperature. It should lose the “transparency” in the thickest part.
Grilling or broiling:
Thick even cuts work best. Meaty fish like, salmon, tuna, mahi mahi work well and are big enough to cut as steaks. Use butter or oil to coat the meat as it is cooking. Exception to the timing would be tuna, as it really loses its character if cooked all the way. So if you want it rare, use a really hot fire very briefly (30 seconds).
Trout and catfish fried with salt and pepper and dredged in flour can be pan fried in oil. If cooked whole, it takes longer so don’t use butter. It burns at relatively low temperatures. Good with Tartar Sauce.
Sole, halibut and flounder fillets are delicate and need to cook fast or they will fall apart– 5 to 8 minutes, again depending on the thickness. I use high heat and clarified butter. (See “Miscellaneous”) Use salt, pepper and flour. It is nice to add lemon juice, butter and chopped parsley to the pan after the fish are removed, to make a Meuniére sauce. Almandine is with sliced almonds toasted in the butter, skip the parsley in this case.
Almost any fish is fine baked in foil with seasoning and thin sliced veggies. Bake at 375º. 12 to 15 minutes again –just guessing.
Use cheesecloth to lower a large whole fish into a pan with 1 - 2 inches of water or court bouillon. I’ve only done this with salmon and I never remember to measure the pan first, or the fish monger never has a small enough salmon. So I always have to cut off the head, tail or both. The idea is to serve a whole, hot or chilled unblemished salmon for a party dish.
I always poke it too much to see if it is done. Anyway, the court bouillon is an aromatic broth, made with water to which you add a fair amount of white wine, some herbs, parsley, bay leaf, peppercorns, chopped carrot, onion and celery.
Serve salmon with Hollandaise or cold dill sauce.
Safety issues: Use fish right away or freeze it. It is OK to sniff before buying. This is normal customer behavior. It should have virtually no odor, or maybe smell of the ocean. Not “fishy”. Eyes on whole fish should be clear.
Always rinse and dry fish and leave it at room temperature for a few minutes and sniff for freshness. Sometimes there is a small amount of surface bacteria, so rinsing and drying will remove this. If in doubt, return it. They will take it back even if you’re wrong, unless you’ve had it lying around for a week.
Wash up carefully, just as with chicken.
Clams should be tighly closed. If they are slighly open, banging them together will induce them to close. If they don't, discard. This is also how to detect "mudders". (empty clam shells which have filled with mud) they make a different sound when clicked together. Same rules apply for oysters and mussels. Mussels need to have "beards" removed before steaming. When cooking shellfish, brief is best. They get tough when overcooked. Watch for shells to open in the pot and then take off heat.