If our little one’s plastic pool is a pain to empty and refill, we usually just cover it and only refill it a couple of times each week. But in doing so, the water develops an oily film. To clean the water without having to empty the pool, use old tennis balls. Toss them in the pool and let them float for an hour. The fuzzy texture of the balls will sop up the film-causing body oils, leaving the water clean and ready for your kids again. Just throw out the balls after each use.