From what I know about aeration the top of the water column does not need to be aerated. There is already enough oxygen in the top of the water column from that water mixing with the atmosphere. You would need to draw water from near the bottom of the pond where the oxygen levels are lower and circulate it to the surface to increase the oxygen in the entire water column. As far as cooling the water I think it would actually increase the temp by mixing the pond, your increasing the amount of water contacting the warm air. That is why a lot of people only run their bottom diffusers at night. Perhaps a 12v inner tube pump connected to some small air stones on the bottom would help? Air takes less energy to move than water so if you could find a 12v air pump of some kind you may be able to run it off a battery longer than a bilge pump.