Can you get a picture? Is it an older, long term captive? Some fish develop thyroid hyperplasia (goiter) and this manifests itself by flared gills, as well as distension of the spot where the gills come together.
Oh yeah - just checking - do you happen to have another bangaii in the tank? These fish are mouthbrooders as you may know, and a male carrying eggs/young is not going to feed and is going to have a distended mouth just like you described (grin).
JHemdal