I just purchased a new home that came with a gunite pool. I have vacuumed all the algae that settled on the bottom of the pool. After vacuuming I noticed staining. I have since drained the pool, and powerwashed it. I am currently in the process of refilling but I still have stains. This is my first time owning a pool and have no idea what the stains are and how to remove them. Please help.