Aww.. fin splitting can be caused by flaring, but since you said he wasn't, then it could be from a number of things such as; sharp/rough/pointy decorations, un-baffled/fast filter, rough substrate where he runs his fins along while scrounging for food, using a net to take him out for cleaning, etc etc. Their fins are like tissue paper, so it happens very easily.
Fins tearing usually isn't due to stress.. they tend to clamp their fins close when stressed.
Just keep up on the weekly water changes to avoid fin rot and he should be getting better. Check your decorations, if you believe something could rip a panty hose, then it could easily rip their fins. Check to see if the filter is too strong (if you use one).
And these types of plastic plants are very notorious for ripping fins.. grr