I would go with the diffuser. I have both in my pond and just turn the fountain on for looks but, I run the diffuser 24-7. I'm sure most of the members that will respond to your post will agree. Useing a diffuser will provide oxegen to several layers of the pond from the bottom up and the fountain will provide oxegen mainly on the surface.I have no idea about the geothermal loop but, I know that aeration has worked well for my pond .