A few things... sand from the beach is not a good thing. There are lots of contaminants in this sand that are unacceptable in a home aquarium. One would think, "If it's good enough for the ocean then it's good enough for my aquarium!" But this simply isn't true! Because we are limited in our ability to filter contaminants and replace minerals/nutrients in the water, we must only use the best resources available! The ocean sand may, in fact, be the reason for your bubbles. I was guessing that it was more of a white film around the top of the water, but after you said ocean sand, now I'd like to see a picture.
I don't think the powerhead will help much. I would look into small HOB (hang on back) skimmers. I see the aqua c remora nano skimmers (5-25 gallons) in the classifieds section of this site and others a lot. The problem is that the protein floats on top of the water, so unless it has to go through an overflow or gets sucked off the surface by a HOB skimmer it really doesn't get mechanically filtered.
I don't know that this is actually protein, though. Pictures and test results will help. If you post those I'm sure more qualified people will jump in with more definitive answers!