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

[mbar]

I don't know why, but rust does not seem to come off with ascorbic acid. You can try using a stain stick, or some fine sand paper to rub the stain. It will take off some of the surface, but not enough for it to matter and it will get rid of the stain. You can also contact Jack's Magic, and ask them. They should be able to answer the question.

[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.