I would be very curious to get the soil tested just under the muck line. I have seen a few folks on here who have renovated a pond like this but had problems with clarity and algae blooms after it filled back up. I am wondering if a high degree of nutrients leaches out of the muck into the subsoil being exposed, and if so, how to remedy that nutrient load so it doesn't return to the water too fast. Now would be the opportunity to check it out.
If high in nutrients, perhaps line the pond with a fresh layer of clay before filling? Perhaps dig a little lower and see if there is less down below?
We never tested the soil but had to leave a portion of the muck because fall rains came and the weather determined when we were "done". And although my son's family gets much use out of the pond, the nutrients have been a problem in the summer both last year and this. He used some Cutrine and knocked the FA back a while this year but still a major problem. He used some pond dye but probably too late for it to keep the regrowth down like it could have if used earlier.
Talked to him today as he and the kids swam and fished in my pond. He is going to get an earlier start on controlling the FA next year in hopes they can use the pond for swimming later into the summer.
But you were correct to anticipate nutrient load being a problem. It is.
