Join Date: Mar 2010
Location: Ontario, Canada
Pooping in the bathtub, or any inappropriate elimination outside of the litter box, is typically due to one of three reasons: (1) medical, (2) problems with the box/litter, or (3) stress. As I'm sure you know, rubbing his nose in it won't solve the first two issues and is likely to lead to even more stress.
For now, I would try to keep him out of the bathroom (close the door), assuming his litter box isn't in there. Also, consider whether there are any issues with the box...perhaps that day they weren't as clean as usual, perhaps you might want to add a box, etc. If it happens again, particularly if the diarrhea continues, I'd suggest a visit to the vet. If there's no medical issue, then you'll have to re-think the litter/box aspect and/or consider what might be causing him stress.