If the vitamin C tablet worked, then it is metal stain. If the cal hypo worked, then it is an organic stain. If it is a metal stain, you can either use ascorbic acid (which is vitamin C), or oxallic acid. I use the ascorbic. Since you only have a few spots, I would try lowering the ph down to 7.2 and adding a sequestering agent like sequasol, jack's magic the pink stuff, or blue stuff, Metal free, any thing that say holds metals in suspension. Try this first - sometimes it will lift off stains that have just started. If it doesn't work, you can then do the ascorbic acid. You can get it at:

http://www.msm-msm.com/store/agora.c...scorbic%20Acid

or:http://www.chemistrystore.com/Ascorbic_Acid.htm