I find that when I have an algae infestation and vacuum the bottom after it settles, the pool is totally stirred up and green later in the day. This means that it takes a lot longer to filter and backwash all the algae out taking over a week. I seem to detect a pale green color entering the pool from the return fitting even as I'm vacuuming? Is my filter old? I was told that I shouldn't need to change the sand and haven't done so in a couple years. Do I need to use a clarifier?