First, Koi are just a colored variant of Carp, and as std Carp do, they will work the bottom sediment and keep it turbid. If it's turbid, weeds won't grow well that grass carp feed on and they too will revert to bottom sediment and add to the problem of turbidity.
Koi are also highly reproductive in this type of environment and would scare me a bit knowing they are there..Best deal would be if the Bass were big enough to consume the Koi. Sounds harsh but would be best if they did get them.
3 big Crappie? Blacks or Whites..???
How are your shorelines for erosion protection from wind/wave action?
If you truly have turbid water, find the source for sure before wasting money on Alum treatment, you may be fighting an uphill battle if Carp are present and causing the issue.