I agree with Ira.
If you have a filter, or bubbles, breaking the water surface it will be ok but if you have a calm surface, the oil slick will be interfering with the oxygen.
I seem to remember someone saying they thought it was due to too many nitrates (or ntrites, I always get them mixed up).
Get some paper towel and carefully drape it on the surface. I should suck up the film. Might take several pieces of towel, depending on the size of the tank.