fish choking?

It looks like one of one cichlids (zebra) is choking. His mouth is going wide open and it looks like he is trying to spit something out. Maybe he swallowed a rock. Any suggestions?

I just pulled him out of the water and I did not see any obstructions in his mouth. However, he is still making the wide mouth movements.
Btw: The tank is a well established tank that I've had for four years, water parameters are fine, there is no overstock and it has double filtration.