Powerhead is basically a lil fan that suctions to the side of the tank or by magnets. It creates water movement in the tank so there's no "dead spots" so everything gets nutrients, oxygen, etc. especially the stuff that can't move on its own like live rock, sand, corals, etc.
Well, we know you have a built in overflow (from another thread)! Here's a pic, the sump/refugium doesn't have to be exactly like this & the protein skimmer may not be in that exact location or be that style. My protein skimmer is in the area w/ the return pump, I don't have a light source for the sump/refugium yet or Chaeto (plant material), I plan to. I don't have a bubble trap or heater (yet). I'd like to get a submersible one. I only have a HOB
in the DT (display tank) kind.
I got this pic from another forum. I can't find the exact link as I'm looking at it on "images" on google on my phone.