Dirty filter and protein from fish food leads to surface Scum. Some say higher level of Fe dosing contributes to this problem, I dont agree. I just installed Eheim 350 skimmer yesterday and to my surprise with in 5 minutes tank is sparkling crystal Clear. I never had the problem with Surface Scum till recently as I started to feed a lot for growing Discus.
"The future's uncertain, and the end is always near."