Can someone give me a short primer on how fish is "graded"? For example, what is the difference between sushi grade fish and non-sushi grade fish? How hazardous is it to use high quality, non-sushi grade fish for sushi?
I am often in Spain for an extended stay and sometimes I go to the fish market, which has incredibly fresh, to buy fish to prepare sushi at home. Things are never labeled as "sushi grade" and I don't think most would know what that was if I ask. So are there some basic questions that I can ask to determine whether the fish I get can be used for sushi/sashimi?