It's not that fish completely lack the ability to digest mammal protein, and or fat, the main risk with most tropical species is the accumulation of fat from these types of feed stuffs. Most tropical fish species require omega-3 (n-3) & n-6 in their diet, and excluding or limiting these types of fatty acids, and/or replacing them with saturated fat (even if the meat appears lean, it will contain a certain degree of these fats) can pose long term health issues with the fish. Also, if too many of these types of foods are offered on a regular basis the fish can end up suffering from fatty acid deficiencies. Open up a fish that is fed hot dogs on a regular basis & I can assure you that you will see excess lipid deposition & necrosis of the liver. Over time this condition will shorten the lifespan of the fish, sometimes by a considerable amount.