Russ, we started with 100 6" - 9" CC in our 1 acre pond. Last year we removed 36; with other losses I estimate 40 to 50 remain, their weights ranging from 3 to 6 lbs. We have NEVER had any turbidity problems that were not directly attributable to runoff from heavy rainfall. My Secchi depth is usually between 2'6" and 2'9"; I have never measured it to be less than 1'7" (I check it, on average, only every couple of weeks during the growing season).
Note that we have always had a lot lower density of CC than 75 in 1/3 acre.
I guess that CC IN SANE DENSITIES (under say 500/acre) will not cause turbidity problems if they have enough to eat - they only stur up bottom sediments if they are very hungry and have to live on BG mouth-sized invertabrates in the muck. I have always fed my CC (when the water temp was > 60 deg) at least a little. Take a look at some Aquaculture CC reports to see the affects of much denser populations. The commercial catfish farmers really pack 'em in, but they also provide plenty of feed.