Odd question, with the tank that has Cinnabar in it, do you feed the corys with the lights on? Or do you have a blue light or something you turn on at night?
Bettas hunt by light, so if you have the lights off in the room they are in as well as in the tank, he shouldn't be able to find the corys food. I'm not 100% sure how corys hunt, but I know they are more active at night and when I drop a piece of food on the other side of the tank, they seem to "smell" it out.
My betta also loves to pick at the corys sinking pellets haha, but if I feed when all the lights are dark, he doesn't seem to notice it.
Just a thought, in case you haven't tried that. :)