Allmadness,
Based on the preliminary info you've given us: For algae control you should have at least 6 of those 4 port diffusers assemblies spread out throughout the pond making sure to get at least a few of them in the deepest portions of the pond. You should put a ball valve on the air supply line to each diffuser assembly or dedicate a compressor to each diffuser so that you can turn off the deeper ones in the winter to prevent super cooling. I would also attach 8-12" horizontal extensions on the air stone ports with vertical elbows such that you space the diffusers further from the cross member. You will also need a coupler with a 3/4" female pipe thread to put on the elbow to receive the diffusers that I'm about to recommend. Then, rather than putting on air stones, you should switch to a fine-bubble diffuser like the membrane types made by EDI
http://www.wastewater.com/aeration-products/disc.htm you can get them out of USA Bluebook for about $16-$28 each depending on if you want to use 9" or 12" (take a look at the spacing between the parallel ports to determine what size you can use). Note: Fine-bubble diffusers are much more efficient than air stones in oxygen transfer and mixing.
In the scenario I described above you would need at least 4 CFM per 4 port diffuser assembly for a total of 24 CFM. This equates to 5-6 1/3rd horsepower compressors (woble-piston style)for a total of 2 horsepower. Be sure to use an oil-less woble piston style rather than rotary vein as the pressures will be too high for rotary vein. I recommend a Thomas 2660 compessor because they handle higher pressures and are durable but they are incredibly difficult to get a hold of these days (although we are expecting 100 of them on Tuesday). In any case be sure to look at voltage, pump curves and ability to restart under pressure when looking for a compressor.
All these parts + additional feeder tubing and connecting components will run you somewhere between $3K-$5K depending on the shape of your pond and feeder tubing requirements. Although I'm baffled at some of the deals I see on this site sometimes so it may be less. I hope this helps. Good luck.....
