Yellow Staining, Acorbic Acid Works Well, BUT.....

[waterbear]

People have used citric acid, which is also less expensive, and it seems to work but just takes somewhat longer. I know that oxalic acid is also a reducing agent so in theory it should work, but I don't know how well.

FWIW, oxalic acid is what is used in United Chemical's Pool Stain Treat and Pool Stain Treat spotting bags. I can't really tell a difference in how fast it works compared to Ascorbic acid but Citric does seem to be a bit slower. However oxalic acid is the most toxic of the three with ascorbic acid being the least toxic.

[mbar]

Make sure as you raise your chlorine levels you add more sequestering agent if you see stains coming back. Add the chlorine slowly - that's why I like to add bleach. You can add it in small amounts till you get it where you need it to be.