Welcome to Aquarium Advice, thartley!
Actually, a powerhead *alone* never (as far as I know) adds air to the water stream. You need an air pump connected to the air tubing, and then that will add air bubbles to the water stream. These can be bought for under $10 at your LFS
If you think about it, a water pump does not create the negative pressure you'd need to suck air from the air line tubing; the water pump simply moves water from one location to another.
I have this sort of a set-up on one of my pleco tanks. It's really a great way to oxygenate the water; and I think it looks pretty cool to have a spray of air bubbles coming out of the water pump.
One recommendation, though... The undergravel filter is an outdated technology. In the 80s and 90s, UGFs were the way to go, as they create a large area of biological filtration under the gravel. However, this area must be cleaned periodically, and requires breaking down the whole tank to do so! If you can afford to buy a Penguin or Emperor HOB
filter, or a Fluval or Eheim canister filter, these are much better filters, and will provide biological filtration on the biowheel (in the case of the Penguin and Emperor) or on the filter popcorn (in the case of the Fluval and Eheim).