Common mistake, but plants are not moral subjects. If you harm any animal, even an insect, it will respond in ways that you or I would; fleeing, retaliating, or generally just panicking. I think you already understand that plants do not (although they do have biochemical adaptations to sense and respond to stress).

Nobody is saying that fish are moral agents that can empathise with other beings. That doesn’t man that they’re not moral subjects; the ability to understand that one is causing harm is not a prerequisite for the ability to suffer oneself. I think everyone knows this intuitively, but it does feel good to have our less moral habits be justified by memes that we would otherwise find to be illogical.