WOW, I can't believe how WRONG that video is on so many points. But, I don't see them use a power head anywhere in that vid except for mixing the saltwater.
As Doug said, it's just to move water around. Usually you use two or more (depending on tank size) pointed at each other or to create an X pattern. This gives you a random flow of water as you would find on a reef.
Now you also need to partial water changes every week. Using a small powerhead in a bucket, trash can, etc. is the best way to mix the salt with the RODI
water. That is in addition to the PH in the tank.
Please allow 3 - 4 weeks between each fish addition to the tank to allow the biological filtration a chance to catch up to the increased bioload and stabilize.