Johnny:

Largemouth Bass (LMB), Northern Pike (NP, or Chain Pickerel (CP) would all help with controlling the koi. One NP might be enough. What species of fish you use will all depend on your goals for the pond.

Regarding your aerator, make sure that the diffuser isn't placed directly on the bottom. If it is, it might be contributing to the suspended sediment in the water. Raise it up a foot or so from the bottom, or put something under it. I've seen 24" to 36" square 1/4" thick plastic used with a single diffuser was attached to the center of it.

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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).