Is it possible the full time aeration and fountain are putting too much oxygen in the water, and that contributes to the algae?
I don't think so.
The algae only utilize oxygen at night. It does most of its growth during the day using sunlight, water, CO2, and nutrients. Some plants do "grow" a little bit at night and consume oxygen from the air or water, depending on whether they are terrestrial or aquatic plants. Even without the aeration, there should be far more oxygen in the water column due to the plants oxygen production during the day, relative to their modest needs at night. The exception is when some outside force causes a significant low oxygen event in the pond.
The only contribution I could see from the aeration, is if it is stirring up bottom water and getting some of the organic muck into the water column. As that muck decomposes, it would be contributing some usable nutrients back into the plant life cycle in the pond. Even if it does make his algal bloom a little bit heavier, I would think it would still be a net positive. You want the organic muck to break down, rather than accumulate on the bottom of the pond.
