Quarter sized oval shaped ringed rust stains on bottom of pool . . .

[chem geek]

I contacted Jack's Magic and I must say that these guys are quite knowledgeable about stains (even knew some chemistry, which surprised me). Anyway, for rust they recommend an oxidizer first (they use something that sounds similar to a non-chlorine shock like KMPS except higher in oxygen content) and then a stain remover that sounds like dry acid (possibly sodium bisulfate). I'll get some of their products and see what happens and let you know. I suspect that ascorbic acid just isn't strong enough by itself.

Richard

[waterbear]

If ascorbic acid doesn't work on rust then oxalic acid might. It is a commen treatment for rust stains outside of swimming pools and is the ingredient in some pool stains revmovers such as 'Stain Treat'.
Richard, Jack's Magic knows their stains! They don't list the ingredients in their MSDS, however but I suspect that the oxidizer that they use is sodium percarbonate.