Fish stock or "fumet", which means aroma in French, is a very simple and quick base for various sauces, cooking liquid to poach fish, aspic or Japanese and Korean consommés. A lot of recipes like the consommés and aspic require a perfectly clear fumet. But, most of the time, even with the best recipe it will turn cloudy, like on that picture below. Why is it happening?
Well, the first cause may be that the fumet has been cooked for a too long period of time (20 minutes is best). The second is that the fish stock was boiled instead of a steady simmering. Boiling or overcooking have basically the same effect on the liquid. They dissolve calcium salt from the fragile fish bones in the stock. These salts will make the final liquid cloudy and give it a chalky taste.
Note that these salts cannot be removed at a later stage, when performing the clarification of the fish stock for the purpose of an aspic or fish consommé cooking.