I do know many local golf courses that have FA problems in their ponds go to spraying the grass in that water shed for the same reason. What they spray I do not know.

I too find a high amount of phosphates in the water that I fill my pond with. I use calcium hydroxide to tie up the phosphates. This is not a perfect science how much to use but you can kill fish if the PH shifts to much.

If you limit the soluble nutrients the FA needs to work to get food.

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7/8th of an acre, Perch only pond, Ontario, Canada.