I have a 20+ year old superpump and have never had a problem with it. The inground pool,with a clean filter,opens at about 8 PSI. It used to require backwashing between 17 and 21 PSI. Now it requires backwashing between 12 and 15,and the pool is growing algae much quicker. Do I try an impeller first,or is this indicating a worn out pump? Thanks.