I was getting that on my 29gal tank. It's over-filtered, but because of the way the tank is built I wasn't getting enough surface agitation on the far side of the tank. I was getting an oily surface, and used paper towel, but it always came back. So, even though I'm not a big fan of bubble wand/disks/stones, I went ahead and added one. Bingo! No more oily film, and the fish seem to like the bubbles.