Where do I buy Phoshonic Acid - and do I need it?

[waterbear]

phosphoNic acid is not the same as phosphoRic acid! Most seqesterants are based on derivatives of phosphoNic acid. If you are putting in water with a high iron content then a sequestering agent is a good idea. The Purple stuff is a decent product as is Proteam's Metal Magic. There are others equally as good. If you are not having a stain problem then all you probably need is a maintenance dose of the product and not the full treatment amount. Since you already have some iron staining in the pool you might want to do a full treatment and then treat with Ascorbic acid (this or oxalic acid is what the majority of stain removal products are; ascorbic acid is much less toxic.) Send a PM to Mbar for details, she is pretty much the forums expert on stains in pools from what I have seen!