they are a submersible pump which you put down in your tank to move your water around in your tank, great investment!
you should use some to move water, and eliminate any still spots
what you want to achieve is movement of at least 10x your gallonage per hour, in your tank, you want between 600 and 1ooo gallon per hr movement in your tank, use at least 2 heads, so you can eliminate any spots without movement
tehy range in price from $15 upto and over 100 each, just do not by a rio when you do get them, for cost and all, I have had good luck with hagen and aquarium systems(maxi jets) powerheads
when you add your sand, it should be 3-4 inches minimum, if you are not using lr, go at least 4 inches, if you are using lr and a real fine grain sand, you can go a little less
calculator try this for your sand