That is a lot of water input and flushing for the 0.75 ac pond. If the incoming water is usually that color or darker, IMO an alum treatment would only last until the next major influx of water. Does the pond have submerged vegetation. Vegetation may help mimimize the turbidity. Abundant vegetation tends to have that affect on suspended solids.
I have a feeling your opinion is probably correct. I'm hoping some of the other experts see this and give their opinions too. As far as submerged vegetation, there is some, but it's not abundant.
