How To Clear Up And Clean A Green Swimming Pool. Unfortunately, there is no magic way to turn a green or black pool clear overnight ! It usually takes a lot of time, effort and money to clear up a typical “swampy” green pool. It is certainly best if you never let your pool water get this dirty, however most pool owners experience a “green pool problem” from time to time. Although it is not easy work, there is a proven method of clearing up a green, black or very dirty pool in the shortest time possible.