My guess on the blue gill floating up would be the aerator getting fired up or even the sand beach work. They tend to get worked up or stressed easy and if weak to begin with will just flip over on you.
Your alkalinity is low. I am blessed here with good water and good soil. Its nice to have 150-200 alkalinity. 20#'s per acre of hydrated lime will do the trick for you but you need to add this slow or you will see more floaters.
If it was me I would try to get my hands on 50#s of alum and 100#'s of hydrated lime then 20#'s of sodium bicarbonate. Mix the alum and lime together in a bucket then pump or throw it into the pond. With good aeration current it will push it though out the pond in an afternoon.
Leave this for about a week then put in the sodium bicarbonate. This will balance the PH and any free acids (alum) or bases (lime) and clear the pond nicely. The only reason I would say to add this is if there is iron in your sand and not just quarts silica. The brown bubbles has me thinking this.
If this was my pond this is what I would be doing.
Cheers Don.
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