Hello- have 18500 gunite pool. I had some brown staining on steps, so tried a vitamin c tablet and worked great. So I dropped a couple on steps and went to do other stuff and forgot about them. The filter turned off and in am I noticed a dark brown/black stain where tablet was. I tried scrubbing and will not come off. Worried it oxidized or burnt gunite. I am wondering if could be scrubbed out with pumice stone or acid direct on spot. Do not want to create additional problem. Water is clear and chemicals balanced- FC was around 1ppm- slowly raising. Any thoughts.