Assuming a relatively triangular, square, or round 1.25 acre pond (not something odd like a strip mine or "V" or "W" shape) with an average depth of 4.5 feet...I believe the minimum number of diffusers would be 4 single 9" heads, all equally spaced to be at the edge of the 5 foot depth areas. This should get the pond water turned over in a 24 hour period. I would feel better with 6 heads (8 if you tend to overdo things like me) because of the shallow water depths in comparisons to the overall surface acreage. I would say this would require a pump that could put out 1 CFM per diffuser. 6 CFM would be in the neighborhood of a 3/4 HP rotary vane pump should you decide to use that style of pump. Being such a shallow BOW there are other options that could work. Diaphragm pumps in particular, but I am most familiar with the rotary vane type.

I have no experience with surface aeration which may be advantageous over bottom diffused in your situation. I'll leave that to other PM members with such experience.

Fish on!,
Noel