clown swimming down, belly distended

[th.one]

Have two clowns (added 1 month ago) to an established tank. Both are active, playful, great eaters. A couple hours after today's feeding one of them is swimming downward struggling not to surface. His belly is distended/bloated. It's tough to watch. Could this be a bladder issue? Too much air swallowed? Impaction? Thanks!