Some of the most mentioned diffusers here at PB are the Vertex and Matala brands. I know you can get the Vertex models from Hoosier Pond Pros...
https://shop.hoosierpondpros.com/Aeration_c3.htmThat's where I bought my diffusers and weighted hose.
The Gast pumps are also a common name thrown around here. My Gast/Vertex system has been in operation for about 4 years with no complaints.
I have my 3 diffusers set off the bottom about 3 foot in my 1/4 acre pond in 6, 7, & 10 foot water (to help reduce the tendency for my pond's muddiness). I have no thermocline with this set-up so the water is getting turned top to bottom, but my small pond is basically the ideal bowl shape for this to work well. I think a larger pond would want deeper diffuser settings and further spacing between each. I imagine 2, spread out, in the deepest area, 1 closer to the middle depths and another near the shallow end in, or right above, the average depth.
Consider running an additional diffuser, or two actually, as the 3/4 hp Gast should be able to handle it given you run the 5/8" line.
