Do you have anything skimming water off the surface of the tank? LIke an overflow or a surface skimmer? These will help a great deal since they pull water off the top of the tank surface thus taking the oil slick with the water churning it up in to the regular water coulumn for skimmers that dont take from the top of the water to process.
If you cut back on your feedings this should help reduce the oil slick as the slick is composed as was said of DOC
's in the water. Increased surface movment of the water. (strong rippling) will also help. To remove the slick take a cup and skim off the surface of the water. The resulting water taken out will be very dark and you will be able to see the DOC
's collecting to the side of the cup.