Hello, a couple of month after my pool was built in 2005, I noticed a small rust spot at the bottom. The company sent out a tech, He said that is was nothing to worry about and he then submerged a 3 inch pvc pipe into the pool down to the spot and poured what looked like an acid into the pipe. When he was finished, the stain was gone. the pool has never had another rust spot, but I have always wondered what type of acid or chemical he used. Any ideas? The pool is a salt water, pebble-tec. thanks, jerry