If you do not have a metal or staining problem then you really don't need the purple stuff or any other sequesterant. Jack's Magic is one of the better lines out there , IMHO, when a seqesterant is needed. Mkamp1515 is right that it is HEDP, a phosphonic acid derivative that can form orthophosphates (algae food) in your water but very few people ever have problems with algae simply becasue of high phosphate levels.....mine have been between 2000 and 3000 ppb and I have never had al algae bloom! A much better way to prevent scaling of your salt cell is to maintain a proper water balance, don't let your pH get above 7.6 (high pH promotes scaling), and watch your TA and calcium hardness. If you have very hard water (high calcium hardness), then it might be of some use in helping keep the scaling down since calcium is a metal and is seqestered in the same way as copper and iron by seqesterants (but ususally not as much).