I have a barbs tank and my barbs do this quite frequently. From what I've read on this subject and from what I have observed this is a behavior displayed when they are resting. Motionless with thier heads pointed downward. if you startle them than they wake up and start to swim again usually only to do it again in a few minutes. It's nothing to worry about. Apparently this is considered normal behavior for barbs in a resting state.
Marinemom
Marinemom