Any suggestions on the most effective/cost effective natural bacteria muck reducer?
You will need to get O2 to where you want the muck removed in addition to whatever you are planing to add, if anything. Plan on aeration to pull O2 down to the muck layer to help digest the organics. There are a lot of resources here to help you choose and install a system.
That is what I always wondered. Why isn't there some natural form of material to dissolve the muck? That don't cost a arm and a leg.
Oxygen stuffed in the right spot tends to be generally free ;-). It is getting it there that is not. But if a pond has a good deal of muck build-up, it is time to get aeration anyhow for the benefit of the fish and to prevent a fish kills when the weather turns quickly.