That may be the case in some instances, but the real reason is regulatory in nature. You can walk into any pet store and buy antibiotics over the counter that are identical in every way to that which you get by prescription at a pharmacy. The only difference is that little disclaimer "not intended for human consumption".My guess (and it is just that), is that there are things in the fish food that humans should not be eating. If the fish eat it, their bodies contain it, and the fish then becomes something that humans should not eat.