Fermented black beans, essential to many fish recipes from China, are heavily salted black soybeans. Their earthy, intense flavor and aroma isn't easily duplicated by any other ingredient. Black bean sauce (easier to find than the fermented beans) is the best substitute. It is a pungent brew of fermented black beans, often with ginger and peanut oil added. An unseasoned sauce is best (try Kame brand, $3.50, www.ethnicgrocer.com).