I calculate the volume of your pond to be roughly 4.5 million gallons. With a single diffuser unit, you should be turning over the entire volume about 1.13 times per day. This should be more than enough for your pond. As for your compressor pushing air down 13' it should be reading on a pressure of about 6psi. That 3/4 should be running fine at that pressure.
Adding Sidekick to your mix helps keep the copper in solution especially if your water is hard. You should get as you mentioned, 2-3 months control with this mix.
