Welcome to the forum,
I am new as well to the pond management thing. But my understanding is that sandy soil will not help you any, also I read on one of the forums about a "Bucket test" where you put some of the material in a bucket and add water to the top let it sit outside and see if rain can keep up with evaporation and leakage. (May want to do some soil tests around your property before getting your heart set on a location and it turns out to be a hard soil to work with, also causing you to truck in loads of material at a hefty cost.)
Someone with way more experience than me will be along shortly I'm sure to give you some good insight.
I am by no means a pro, also just learning my 2 cents followIf you use google you can just type in "bucket test site:pondboss.com" it pulled up a lot of hits and limits it to this forum.
I also read someone had a way to test how deep the pond was without getting wet it involved a float, a weight and some line. The actual setup escapes me but I imagine it is based of a sliding bobber theory.
By the looks of your pond you are experiencing similar issues that I am the thin film I believe to be pollen as mine cleared up naturally after that season passed. The darkness is caused by what I think is to many nutrients and dead leaves making the muck layer thick. My plan for my property is cutting out a good amount of the newer medium to small sized trees. I like the look of old growth trees better personally. Then I will also be aerating(for muck control) and adding water(to fight a growing weed issue)
I am working on cutting my trees back currently.
Hope this helped some and have a good day
