For those of you who have a deep pond, seven feet or deeper on average, you need to use bottom aeration in order to provide a healthy pond ecosystem where plants and fish and other animals can live in a balanced environment. Surface aerators only aerate the top layers of the pond, and if your pond is deep, a display fountain will not aerate the bottom layers.
Bottom Aeration: 
Three parts are combined for bottom aeration: an air compressor, a diffuser, and tubing that connects the air compressor to the diffuser. The tubing ought to be weighted so that it sinks to the floor of your pond. The compressor needs to be housed to prevent burnout or shorts due to bad weather. You can buy housing already made, or you can make your own. If you have a boat house or other storage shed already on your property, you can use that. Bottom aeration provides no display.
But Where is the Fountain?
Quite a few and perhaps a majority of people also want a display, and in that case you can combine a bottom aeration (diffuser system) with a pond fountain for display (plus agitation at the water's surface). A surface aerator floats on the surface (with a pump attached underneath the float and a nozzle on top of the float).
Since you are already aerating your pond with a diffuser system, you can concentrate purely on display for a surface fountain. You can buy a budget fountain or an industrial fountain and, if desired, focus on large displays. Large displays are typically accomplished by using lots of horsepower or using smaller holes in the nozzle (to provide more lift and width). For the largest displays, you should take a look at Scott Fountains. The 1/2HP Jet Stream nozzle, for instance, provides a 35' tall display. The 1 1/2 HP Jet Stream nozzle provides 50', and the 3 HP provides 80'! Yes, 80 feet tall. For those who believe taller is better this is a very good option.