I can't imagine a food less likely to be encountered in their natural environment.
Even many land based animals experience gastric upsets with soy- can't think what it would do to a fish.
tofu while an exellent source of protien is lacking the fiber or roughage that makes it possible for the fish to move things through.
I suppose if you have a large fish and added tofu as a small, say 10% of a particular meal, additive to your regular food you could get away with it from a regularity of bowel movement issue.
but as to the fish's ability to digest the components of tofu itself I don't know.