We have a chlorine machine that makes chlorine from salt. We have had this for about 5 years with no problem and have just had it serviced. The chlorine it makes is unstabilized so I add stabilizer. That said, heres my problem. I've got a stain on the plaster that is a dark yellow. It has not responded to three algaecide treatments. I have taken a full set of readings, keep in mind the CH is not usually so high, I've increased to fight the stain. Thanks for any advice.