Filleted fish
Cooking times for fish vary depending on the size of the fish and the cooking method. While there are methods for estimating how long a particular fillet or fish will take to fully cook, the only way to know for sure is to measure the internal temperature.
Method
The prevailing method for estimating cooking time for any fish is to measure the thickness of the thickest part of the fish. Once thickness is obtained, plan for 10 minutes per inch. While this process is neither scientific nor precise, it can give you an educated starting point.
For pieces of fish more than 1/2-inch thick, use the above method and flip the fish halfway through the estimated cooking time. For whole fish, insert a knife next to the backbone and attempt to separate it from the bone. If it comes off easily, it should be done.
Doneness
Differences between accepted internal temperatures and enjoyable texture and taste affect our cultural interpretation of doneness. When properly cooked, fish should be flaky and opaque. Firmer-fleshed fish like swordfish may not flake easily; do not let this concern you.
The USDA recommends that fish be cooked to 145 degrees to eliminate potentially harmful bacteria like E. coli, listeria, and salmonella. The best way to check internal temperatures is to insert an instant-read thermometer into the center of the thickest part of a fish.
Contrary to popular thought, raw fish "cooked" with highly acidic liquid such as lemon or lime juice is not actually cooked and may contain harmful pathogens. The acidic juice makes fish flesh appear flaky but because no heat is used, pathogens may still be present. When preparing ceviche, only purchase fish from trusted sources that follow USDA guidelines for selling fish for raw consumption.
Techniques
Be sure to get the freshest fish from a trusted purveyor, because fish, more than other meats, is particularly perishable and should not be held in a home refrigerator for more than a few days. Before cooking, let fish warm to room temperature for about 30 minutes. This will help ensure consistent cooking and a faster cooking time overall.
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