Yes it can, depending on the quantity ate, the size of the dog and just how its body reacts to it. You shouldn't feed chocolate to your dog. They have a pet friendly alternative - carob! Looks just like chocolate, dogs love it and it's completely safe! However one night a few years ago I discovered my German Shepherd had eaten my entire block of Cadbury chocolate - but she was completely fine. It may have something to do with her size, but please don't even risk it! Not sure about the fish thing, my dogs have never liked it.