I know each dog is different but Fletcher can not tolerate any fish oils, capsules, Salmon oil, any of it. However, he does fine with canned sardines and I used to also give them jack mackrel, when it was cheap. He has no problems with actual fish. I also have a friend who has a cat who can't eat any kind of fish cat food, but does just fine with plain cooked salmon or tuna. Something in the processing, I guess