Fish sometimes gives my dogs loose stools. I'd switch to something like chicken and see if it improves. If the stools are mucous covered, he may be out of balance with bad bacteria and may need some probiotics or even Flagyl from your vet if that does not work. I just posted a link elsewhere about a product I use for my dog with MVD and IBD who gets diarrhea often. Here it is if you are interested.
Acidophilus for Dogs & Puppies, Dog Probiotic, Kosher & Lactose Free - 16 fl. oz. - Dynamic Health