I have a contract on a house that was built in 1972. Not sure when the pool was added. The pool is 30 x 50 with a vinyl lining and mesh top. It hasn't been used in 4 years but there is still plenty of water in it (very nasty water) but enough to keep the pool from collapsing. Is it still possible to cleanse the water or must I drain the pool and refill? I've never owned a pool so any help would be appreciated.