The fish in that tank are not getting along, and it is not an example of appropriate stocking.
First, what makes you think the tank is "overstocked?" To most, and I am assuming to you, "overstocked" means too many fish. There is no list of the fish, and it is impossible to tell from the video. I agree the tank is inappropriately stocked, without question. The Pearl Gourami are highly stressed. They only appear for a few seconds, but their behaviour in those few seconds is certainly not normal and is a sign of stress. I suspect it is the gudgeons, from what we can see of the fish species.
Forget any videos on YouTube unless they are posted by a knowledgeable hobbyist, one you know is knowledgeable. The one here is not, clearly. It takes years of observing fish, and thoroughly researching their requirements, to be able to quickly spot the trouble that is evident in this tank.