this sort of thing happened to my octospawn. it was big and happy. came home and it was all closed up. couldn't figure out what happened. So I started blaming my peppermints. so last resort, not having much faith in the fix, i decided to move the octo to the opposite side of the tank from where the peppermint resides. I'm going on a month plus, and the octo has rebound, and is growing again. Not nearly what he used to be, but he looks happy once again. I tried catching my pepps to put them in the fuge, but gave up on that, no patience.
so since moving the octo, everything is happy again. for now.