A diffuser is just a large bubbler kinda like an air stone for an aquarium. One will move approximately 2500 gallons per MINUTE in 8 foot of water. How many gallons does your pool pump push? I bet it's ALOT less than 2500 GPM (more like 2500 gallons per HOUR) and you would likely need 2 diffusers for your pond at a minimum.

A pump system like you describe used to create a fountain has some benefits with respect to surface aesthetics and some oxygen levels, but those benefits will be minimal compared to a full blown aeration system.

Along the lines of what Highflyer stated...you need to move alot of water and get it in contact with the air. A diffuser system does not spray the water into the air, it just moves it to the surface so that it can absorb the oxygen.

Welcome to the forums! You've come to the right place for al things ponds.

Fish on!,
Noel