There are all kinds of powerheads, from expensive wave maker systems for reefs to cheap ones designed for use with protien skimmers and UGF's (under gravel filtration).
They come in different shapes, sizes, gph's,and mounting types; from ones that look like fans mounted in the water to mini jets that hang on the back. A 75 gal tank isn't that deep,wide, or long. If it were me I'd go with the hang on Marineland esp. since poop seems to be an issue in question. These jets have a flap at the outtake it can face up or down. I would go up for increased surface aggitation. The gas exchange generally takes place at the surface thru movement, nitra out - oxygen in. The majority of the tanks dissolved oxygen comes from the atmosphere by this method. The Marineland powerheads also have an attachable fine particle filter sold seperate or with the model 60 in combo. This will help draw up the poop and trap it. The filter more than likely won't solve excess poop issues, gravel vaccumming as needed will. The marinelands also have an adjustable air intake valve system that forces atmospheric air into the water which can help greatly to increase dissolved O2 into the tank. If you were to prefer current within the tank for O2 circulation Koralia makes some good fan type heads.