I have an 18,000 gallon white plaster pool that I have teaken care of for the last 4 years. The pool came with the house and it was a foreclosure so I dont know how old the pool is but it does need replastering. Last fall a lot of leaves accumulated in the pool as well as other organic stuff (I have a "fruitless" pear tree that hangs over the edge of the pool at it dumps all kinds of very small pears) that I couldnt get cleaned up for some time and now that I've cleaned the pool there are permanent stains. Brown stains to be exact. What do I do about the stains?