The colors of cooking
When cooking sea food the single most basic indicator of progress is the change in color of the fish being cooked. Whenever I have to teach fish cooking I use the same basic lesson (below) and encourage you to have fun practicing it for yourself.
Any white flesh fish
Select a piece of a white fleshed fish, a piece of Salmon, and a piece of Tuna, and cut each piece of fish in two. Heat a heavy frying pan to medium, adding a small amount of oil.
Place all six pieces of fish into the pan and watch as each piece of fish is heated. The flesh changes color as the heat moves through the depth of the filet. At the same temperature that the fish changes color, the 'glue' holding the sections together has melted away. When you feel you are half way through cooking, remove one piece of fish and cut through the center to see the color changes. When the color is at the point you desire, you should stop cooking and start eating.
REMEMBER! The steam inside gains momentum as it travels through the fish, and will continue cooking after being removed from the heat. To cook the fish through to the center you must stop short, and allow a few minutes on the plate for the cooking to finish.