I do not think 5 koi would muddy up a 2 acre pond. Now if those koi start reproducing and there aren't LMB or another high end predator to prey on the young, then you could have issues in the future.

Koi are the same speices as common carp, just selectively bred over many generations to have unusual coloration and form. Fish for the the same way you would carp. Consider baiting an area with whole sweet corn kernels, perhaps a half handful for a day or two and then try fishing the bottom with light line no weight and a size 8 hook with 3 or 4 kernels covering it.