Swimming would be O.K., I do in mine. But having the ducks in there is just asking for problems. My neighbors pond was muddy to a point too. Once the pair of ducks were removed, the pond cleaned up. I would have never guessed that only 2 ducks on a 1/4 ac pond could muddy the water, but that's the only explination.

That planimeter is more accurate the larger you can make your pond (zoom in).


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[Linked Image from lh6.ggpht.com]
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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).