I think you mean fish don’t express themselves in a way you understand. Some are lone hunters who have to rely on their wits to survive, while some have complex social interactions. Some even pass the mirror test.

I don’t think you should make excuses for why some things deserve life or kindness and others don’t. I think it’s better to just be honest with yourself about your personal biases and say you like dogs too much to hurt them, but that you don’t care as much about fish.