I have a concern with some of my fancy goldfish that seek the surface after being fed. They sit there gulping air in and I am concerned that they may get swim bladder issues from it.

Why do they do this? I have plenty aeration (2 Airstones and a lot of surface agitation). Also it is only 4 of them that do it. The rest just swim as regularly.

The tank is slightly overcrowded. They are in a 140 gal while I reorganize their 190 gal. but nothing extreme. Also they did the same when they were in the large tank so I don't expect it to be down to overstocking.