Stress coat will leave a film from the aloe. You can also get a film from not-fully-cured driftwood, feeding FD krill/plankton, beefheart, liver, oily feeder fish, etc. If you don't have surface skimming for filtration there's an 'old school' way to remove this film. Take several squares of paper towels, drop them flat on the surface of the water, and immediately pick them out. This will blot (wick) the oily film to the paper.