At higher densities, Koi may stir up bottom muck/sediment rooting around for food.

If it's just the two, and they're the same sex, you might be OK.

I know some people do have koi in their ponds coexisting w/ LMB and other panfish.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."