Charlie, I've got exactly the same situation as you as far as pond size and depth. If you want to keep your pond destratified then you definitely need at least as rotary vane pump. The centrifugal blowers that a lot of people use won't provide enough pressure to pump down to the bottom at 8 feet. I bought a similar vane pump as Wrangler man describes, these are made for pressures up to around 15 psi. You will need about 5 psi to reach the bottom and another 5 psi to overcome resistance in your long air line. I had to run about 400 feet of line and used 1/2 inch plastic irrigation pipe, wished I had used 3/4 inch since my pump is now driving 10psi. Would be less with larger hose. I wouldn't use anything but a air stone diffuser. I've tried both membrane diffusers and air stones. The air stones do an obvious better job of producing small bubbles for much better aeration. Membrane units are not meant for efficiency in adding air to water. You also definitely need a check valve right before the diffuser so that the pump doesn't have to drive all the water out of the line every time it has been shut off and restarted. You can find various manufacturers of vane pumps on the internet. A 1/4 hp unit is more than adequate for 1/2 acre. Go to Aquatic Ecosystems web site and look up their technical help on pumps and you'll find good information on how to pick the right size. There is lots of good technical info on their site. They also sell all the diffusers and other stuff you would need. I went the do-it-yourself route and copied what they sold by buying all the parts locally and built a 1/2HP aeration unit. Still cost about $600 but was much cheaper than buying the unit already assembled. I also devised a cheap way to mount the air stones on a weighted stand slightly off the bottom. This holds the thing in place and you need to be off the bottom otherwise the thing will just keep digging its own hole. Definitely an easy do-it-yourself project as long as you size the pump and lines correctly.

Gotta get back to fishin!