The short answer is, you can't. If your pond has frogs in it, that means that current conditions are to their liking. Clean it out and they won't be. Decaying leaves are great tadpole habitat; it gives them a place to hide and a substrate for algal growth on which to feed. Decayed leaves turn into soil that provides a winter shelter for adult frogs. Most wild animals don't like "clean" environments; that's why most man-made habitats have so little biodiversity. If you treasure "your" frogs, leave the pond alone. If you must do some cleaning, the safest (for the frogs) time of year to do it would be mid fall, after most of the tadpoles have metamorphosed and before adults are settling in for the winter. Keep in mind that the frogs may take two years to develop, so there may not be any completely safe time.