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Start by getting as much out as you possibly can. Often it can be skimmed off the top, or picked up by special oil absorbent pads some auto parts places sell. Once you get the quantity remaining down to something reasonable, chlorine will break it down, though that process takes a while, probably a couple of weeks. You can speed that up by using an enzyme treatment. Pool Perfect is a good product, and there are several others that are similar. Often they are advertised as helping break down sun tan lotion.