A couple of months ago I was careless and let the chlorine get to 0. The floor and walls turned yellow. So I brushed the stains off and brought the chlorine back to normal. But the stains keep slowly coming back, though the numbers look good. Is a shock necessary once the stains appear? Is that why it keeps coming back? What ppm will be sufficient? Also, will a shock get rid of the stains that are there now on its own or will I still need to brush it off?