I'm quite certain that the current scientific understanding is that we don't know how consciousness works at all. This is exactly the kind of landscape where one might expect big surprises.

Think about how useless our intuitions proved to be when it comes to other phenomena like gravity or electromagnetism. It took a while to (kind of) figure these out despite them being easier to measure than consciousness.

W.r.t. fish: they act in a way you'd expect a creature to act if they felt pain. If they had vocal cords and thus the ability to scream, would you agree that they feel pain? Same applies to insects.