Stain Removers or Sequestring agents are used to control or remove metals and minerals from your spa. They coagulate the particles so the filter can remove them.  While most stain removers have phosphates in them (remember phosphates are the nutrients for algae) we want to limit the use of these types.  Always look for a phosphate free stain remover.

Note: Also keep your pH and Alkalintiy in balance and it will also help reduce staining. pH should be 7.2 to 7.6 ppm and TA should be 80 to 120 ppm.