Just a thought...if your 1/2 acre pond is about one million gallons and your aeration system has been running 8 hours a day...you should have been turning the pond over about once in that 8 hour run. I would think that would eliminate the possibility of a toxic turn over, but you might have been on the cutting edge. Do you have water temps from throughout the water column?

Wbuffet is likely correct in that the 0523 can handle 3 vertex diffusers...depending on you setup. My 0523 pushes 3 diffusers, but my lines are 5/8" and the deepest setting is about 8 feet. My guess is that it will push 2 diffusers very well, but three might just be on the edge of good efficiency. Either way, I think spreading them out in the pond would still be a benefit.

Fish on!,
Noel