What causes stains and what can be done about them?

First, take a pool brush to the stain.  If it brushes up it is algae or dirt, not a stain.  Stains are caused by metals such as iron or copper which get in the pool from fertilizer, well water, the metal used in pool ladders, lights or heaters.  We can do a test to determine what kind of stain it is and if a stain treatment is likely to be effective.