There is a myth that comes from French cooking that fish stock must not be cooked for more than twenty to forty minutes. As an ignorant young cook, for many years, I cooked my fish stocks for long hours, just as I did my other stocks. As I learned more, I persisted in my very satisfactory habit.
It was not until years later, when I began to work with restaurants, that I understood the shibboleth. French restaurant chefs use the frames of flatfish, flounder and sole, for their stock.