When people make categorical statements like "a fish is not a dog," I want to know what criteria are being used to make that distinction. Some people employ all kinds of mental gymnastics to justify why they are outraged at puppy mills, but okay with keeping a Betta in a shot glass. I am more interested in that thought process than I am in the actual treatment of the dogs or the fish.
Taxonomy. That's the distinction. No one is making up excuses when they say they are different because Physically, Mentally and Scientifically they are different. That's all there is to it.
I don't know why you're trying to argue against whether a dog and fish are different or not, because no matter what way you put it, they are indeed different and are not comparable.