I would really like to make fish fumet and i have a few different questions. 1.) I have read that the best and the only bones to be used in fish fumet are saltwater white-fleshed fish like sole, halibut, flounder, etc. Why is this? 2.) I have also seen some recipes that call for fish bones (no heads) and i have seen some that call for both fish bones and heads. why do some recipes specifically say no heads? 3.) Can i make fish fumet from freshwater fish bones? if so which fish?