limbic system is the center of our emotional reactions... and in humans we actually have a rather well developed ability to supress emotions (to use the rational parts of our brain to overcome fear in a given situation, let's say)

it's really interesting actually. Also the limbic system is really complicated on it's own, as different emotions compete with each other, fear/anger/joy variously release different hormones and try to suppress the others (where try means send signals suppressing the neurons in other areas of the limbic system... the strength of those signals depend on how stimulated an area is in the first place... so how much of a 'happy' stimulus you received... let's say)

it is really cool (not even looking at the 'higher' brain functions, which can imagine stimulus and feed them into the limbic system, getting largely the same kinds of reactions.. just think about giant stick insects :) )

As for fish, they can still be stressed. They still have a nervous system, there are shades of grey between the 'human suffering' and 'bacteria under environmental stress'... just like their are shades of grey between the single zygote (no nervous system, no libimc system) and the fully grown human.... you can't just pretend things are simply black and white.... well actually you can just pretend things are black and white, but you shouldn't...