JM-
I ran the numbers on your pond and with the diffuser at 15 of depth & 400' of 1/2 tubing (300' buried & 100' in pond) your psi requirement will be 7.026 psi. The rotary vane you posted will do this.
Your pond I estimate contains 4,160,238 gallons of water. We size systems to make sure the volume is circulated at least 0.75 times per day. This typically ensures we meet the oxygen demand of most ponds and lakes.
Only one diffuser will be required to meet that turnover rate in your pond. Since you have air left over, you could install a second diffuser without any concerns other than increasing the circulation rate. With two diffusers your turnover/circulation rate will be 2.57 times/day.
As Sue mentioned above, regardless of what diffuser or compressor you choose, make sure your enclosure is well ventilated. These compressors are not big fans of heat. I do recomend you go with a weighted tubing in the water. It is well worth the additional cost in the long run! I'm sure Ted will go over that with you.
