What are your thoughts on this plan: 3/4 acre pond with almost perfect oval shape and max depth of 13 feet. Gast 0523 running 50 ft of 3/4 poly tubing to waters edge then stepping down to 100 feet of 5/8 weighted line to a three diffuser setup in the middle of pond. Three 9” Flexair rubber membrane diffusers. Anything you would add or change?

Only thing I would add is a manifold assembly so that each diffuser had it's own ball valve to adjust the air going to each one, a pop-off valve in case pressure gets too high for any reason and a low pressure gauge to monitor system pressure.

Where the compressor goes has to have lots of air flow to keep the temperature down where the compressor is housed. Also put a large air filter on the inlet side of the compressor. The sintered metal "filter" is not really a filter, it's just there to keep it quieter.

Actually my plan was to have the three diffusers ganged together off one single weighted line.

Then you can ditch the manifold valves, but I'd still recommend the other items. I am assuming when you say ganged together you mean all at the same place in the pond.

Yes all 3 together in the middle of the pond. One other thing I am trying to figure out are the fittings required to go from the 1/4 NPT outlet on the pump to the pop off valve and pressure gauge. Also the transition from the 1/4 to the 3/4 poly and back down to the 5/8 weighted.