Gents,
Thank you for the thoughtful replies. I measured the perimeter on Google Maps, and came up with 0.45 acres. It is about what the owner told me, at half acre. Max depth is 12', so I came up with approx 500-600k gallons, depending on average depth (no way to know that one).

Realistically I would be surprised if there is even 1-2" of clarity right now. Water is light green. Ideally, would like to keep at least a few Koi (maybe 10?), but my main concern is water is so green right now. Can't be healthy! I don't see filamentous algae though. Not that interested in having other fish for angling and such. Going for more Asian feel.

We can laboriously clean out the 4-6" of green muck from the man made waterfall area, but I can only assume the bottom of the pond, which is unfortunately covered with baseball sized rocks, is caked in with muck too. Any ideas if it should be addressed somehow, or left?

Also, was suggested to put in bead filter of some sort and 240w UV light to try to clean the existing water. At over 500k gallons, not sure will help?

Will upload some photos as soon as I can figure out how...

Last edited by NewYorkPond; 12/17/17 11:37 AM.