The most likely cause of your cod being tough is overcooking. You are aiming for an internal temperature of 145 degrees F, so try not to overshoot that temperature by very much if possible. The amount of time needed in the oven will vary based on your own individual oven, and how thick your cod pieces are. Thinner pieces will take less time. So, unless your cod pieces are very thick (over 1 inch/2.5 cm), start checking the cod at the lowest end of the cook time listed in the recipe to avoid overcooking.