Rinse it a million times, and then a few more! I've only used it once so I'm not sure there's actually any way to avoid at least a little bit of an oil slick... I did rinse it well (or I thought it was well-rinsed, lol) but it still did the terrible thing. Didn't cause me any problems besides aesthetic grief, though. The tank cycled fine and the oil went away with paper towel skimming and time. As for methods, I agree with burr740.