Do pond fountains actually aerate the water?

Short Answer:
Yes. Fountains provide varying degrees of aeration.

Detailed Answer:
Fountains move water at the surface and create some oxygen exchange, but they do not circulate deep water effectively. So they are particularly good for shallow ponds less than 6 feet. There are two types of fountains to consider. Aerating and decorative. Aerating fountains move more water and will add more oxygen to the water than a decorative fountain.

Comparison:

• Fountain → aesthetic + moderate oxygen
• Surface aerator → maximum oxygen infusion into the upper part of the pond
• Bottom diffuser → good oxygenation with full pond circulation

Bottom line:
Fountains do aerate and add oxygen to pond water in varying degrees. Surface aerators provide maximum oxygen transfer, and sub-surface aerators provide the best circulation in ponds deeper than 6 feet.