Some more information would be helpful. How long has the tank been running with fish in it? how long have the Angelfish been in the tank? Were they all placed in the tank at the same time? How large are the Angelfish? Does water test zero for ammonia and nitrites and nitrAtes under 40ppm? What products or chemicals other than dechlorinator are used in the tank?
The fact that the Angelfish have been doing well for sometime,and that the Angelfish in question seems to swim and eat? ok when interacting with the others,,, could very well be simply territorial issue. Could also be that a clutch of eggs has been deposited near the area where the fish exhibits the behaivor described. Might look closely to see if there are any eggs on plant leaves,glass,or filter uptake near this area. I am not thinking it is swim bladder disorder ,or it seems fish would exhibit the head up,tail down,posistion more frequently. I might were it me, perform twice weekly water changes of ten gal in this tank for the next two or three weeks and observe the fish. I might feed these fish sparingly ONCE a day for the same duration. I might also see if fish will take some freeze dried or frozen Daphnia once or twice a week. If fish are feeding well,, and the angelfish in question has no trouble staying right side up and or swimming normally when it swims with others,, then the twice weekly water changes along with reducing the feed temporarily,, could benefit the fish more than attempting to medicate at this point.
I would also try releasing the foods offered below the surface of the water as opposed to scattering flake food on the surface. This may help fish by limiting the air they may take in while feeding from the surface.
I believe I saw a power head? in photo of your tank. With the Emperor 400 on this tank,, it is more than enough circulation and perhaps the powerhead is making it difficult for wide bodied fish such as angel fish to hold themselves where they want in the aquarium.
Is also possible as Mitch observed,, that the fish are pairing up for possible mating. If this happens, you may have to place the mating pair in a seperate tank. Angelfish can do well in groups when they are small but when and if pairs form,, they are less and less tolerant of other angelfish that they think may be trying to win the affection of fish that they have selected or would like to. Hope some of this helps. Again ,I would not be in a hurry to medicate the fish but rather try some of the above and see how fish respond.