What chickens can eat & digest fish. Where did you hear that they can't digest fish.Looking at Forco's website, this product looks to be a probiotic. Avians need a specific type of probiotic, giving a horse probiotic to avians is not a good idea. This horse stuff may be doing internal damage to birds. Forco's website also says "There are no supplemental vitamins, minerals or proteins". In nature chickens never eat tuna, why would you give it to them. The chickens may have had issues trying to digest the protein from fish. Feather plucking is a protein deficiency, there is protein in feathers, that is why chickens eat them. Try something more suited for avains, like game bird feed, it is 24% protein instead of layer feed that is 14-16% protein. Or mealworms, around 50% protein.