Fish is cooked when it has reached an internal temperature of 140 to 145 degrees F. Keep in mind that fish continues to cook after being removed from the oven, so it’s better to take it out two or so minutes early. It will finish cooking as it rests. If you don’t have a meat thermometer, look at the color and texture of the fish. Cooked white fish flesh will turn from opaque to white, and will flake easily with a fork. To avoid dry flounder, keep a close eye on it after it has been in the oven for 7 to minutes.