We, as omnivores, have a choice. The carnivores do not. I'd rather not cause more suffering than I have to (since I have that choice) even if there was the potential that it could possibly decrease overall suffering.
I will not go into other problems with fish specifically since it's not on-topic.
Do note that this whole thing is based on the hypothetical of plants being capable of experiencing pain. In reality, they do not possess a nervous system to enable that.
Of course I'd choose to kill an animal if the alternative was getting injured or killed (or starving in some extreme survival situation), but in day-to-day life, I do not see the need to do that.