Forgive the long first post - i have been reading a lot and trying to learn what I can, but we have a real challenge. We are in the process of closing on a home in upstate NY with an approx 1/2 acre koi pond that is a pretty green mess.

Pond is estimated to be 0.45 acres. Has max depth of 12 feet. Per some calculations, it looks to be approx 500,000-600,000 gallons. Pond was built maybe 25 yrs ago. It was originally naturally lined, with an earthen bottom. It is reported to be spring fed and holding water well. About ten yrs ago the owner temporarily drained the pond and lined the bottom with “baseball sized rocks”. His idea was to keep the stirred up soil from clouding the water when the fish swam by.

Fast forward to today - The water is sadly quite greenish with little visibility. Doesnt appear to be any visible filamentous algae, just light green water. Rumor has it, it has been like this for a good 10 yrs. It currently has 100-150, plus or minus, good sized koi and gold fish. There are no filters of any sort and only 2 aerators in the pond. There is a man made waterfall/ river that drains into the pond, and is maybe 40 ft long. There is a pump with maybe 2” pvc line that recirculates pond water to the top of it to run down.

We met with a local pond company who checked it out. On inspection, the waterfall/river area must have had 3-4” of greenish muck all along the bottom, interspersed with the rocks. The edges of the pond had lots of visible greenish or gray muck visible around the edges. All thought to be deadish algae.

We are not sure what best strategies are to clear it? It was advised that the fish population be thinned way down to 20-30 for now, as they are the primary source of green muck. Of course, we have no idea what to do with the fish. Also adding physical filtration of some sort and also a UV filter was recommended too. However, after some research, it doesnt look like any UV or physical filters are designed for a pond near this size? With a rock bottom, I can only assume there are several inches of green goo packed around the rocks on the bottom, like the waterfall area, but nearly impossible to remove. They think the waterfall/river area should be semi-demolished, with the rock bottom removed so the inches of muck can be shoveled out. Not sure what one can do for the pond bottom, or if it can be left?

Any advice appreciated, as it is a great house and the pond environment is otherwise lovely. Just need to get clearer water somehow.


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Last edited by NewYorkPond; 12/17/17 11:41 AM.