IIRC fountains only aerate water at the surface and do not do a good job of mixing water throughout the depths of the pond. OTOH an air injection system (be it compressor or windmill, airstone or diffuser) can be used to aerate and mix water down to the depth you set the diffuser/airstone at.

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
[Linked Image from i.pinimg.com]