I think alot of the problems stem from lack of surface agitation. Try raising your supply up so that it agitates the surface. That will definitely help with the problem of the surface film. Also, I would figure out what your overflow problem is very soon or your pump will burn up on you. The mag pumps don't have much head and the five bends combined with the shut-off valve being half closed is maxing out you available head and will kill your pump very fast. When you add in 90 degree elbows and the dreaded tee-fitting you eat up major head on a pump. The best solution would probably be to add an additional overflow box assembly and plumb it directly to your sump. I'd be willing to bet that your return line just can't handle the flow that your pump can put out. If i'm correct, the mag 24 flows 2400 gph max, which is way more than a 1 or 1-1/2" return pipe can handle through gravity and siphon alone.