I've never seen any of my fish fart, but my old gourami's would have the slightest squeek of air coming out. At a glance, its hard to tell between a burp or a fart.
my african knife does this. it fun to watch because some times he will do it super slow, and others you cant even see him do it, you just hear water hitting the lid.
mine are trying to gulp from the surface but only one makes it the other three seem to give up 3/4 of the way to the top. if they dont make it is that bad will they die????