I am testing my water myself with Ben's test kit. I recently decided to have a pool store do a test for comaprison. The pool store suggested I add Jack's Magic ("Purple" for salt pools). I know they want to sell me something but I may be in agreement with them on this. The product claims to prevent stains - which I am concerned about because I top off the pool with high-iron well water.

Should I use a stain prevenative? I already have two small brownish stains near the main drains I can't get rid of. Jack's Magic says it contains Phosphonic Acid. If so, where can I get this stuff without paying $18 a quart? My pool needs two quarts per application.

I have a 20,000 gallon in-ground gunite pool with Diamond Brite type finish.

Thanks,
Mike