Once to Jewel when she was not eating when she was a puppy. This was after the vet gave some meds to her which decreased her appetite. The vet did not tell us of this side effect. Needless-to-say, we do not go to that vet anymore.
My bullmastiffs used to love tuna, but the vet warned me about the amount of mercury in it. I think we'll use it as a topper (very small amount), but we haven't used it on Mabel's food yet.
I did... when she was pregnated... instead of mixin canned with her kribble i did tuna or deboned chicken with her kribble she liked it but not all the time like 3 times per week tops..