Bake frozen fish for 20 minutes per inch at its thickest part, turning it halfway through the cooking time. Bake for an additional 5 to 10 minutes if cooking in foil or a sauce. Fish is cooked when the color changes becomes opaque and the flesh is flaky.
Cooked fish also has an internal temperature of 145 degrees Fahrenheit. The United States Food and Drug Administration recommends thawing fish before cooking it. To defrost fish quickly and safely, seal it in a plastic bag and place it in cold water or use the defrost setting on a microwave oven, stopping the defrost cycle while the fish is still icy but pliable. Bake defrosted fish for only 10 minutes per inch at its thickest part, turning it halfway through the cooking time.