I'd kill it and start over. Have a stocking plan before you start. Go here http://www.acme.com/planimeter/ and you can figure out the size of the pond.

Stock RES in the new pond, they will help with your parasite problem.

Drain as much water out of the pond as possible - you'll need less Rotenone that way.

www.hoosierpondpros.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).