Epsom Salt?

[waterbear]

The idea is to raise magnesium hardness since magnesium scale does not get hard but stays soft and rinses off easily.
Lo-chlor makes a magnesium additive for salt pools called salt cell protector for this reason. Whether it works I could not tell you.

[Mark_WATERMAID]

I know that this post is pretty old but I think that I should throw in my two cents. Wateramaid recommends that customers add 50 lbs. of magnesium chloride to their pools every year. Calcium is attracted to the cathode plate of a salt chlorinator and can form hard scale. Adding magnesium softens the scale and makes it mushy. Normal water flow tends to flush the scale off of the cell plates. It also adds hardness to the water without using calcium. This once again reduces scale formation. Magnesium chloride will not harm a Watermaid cell. In fact it will increase the cell life. If you let a cell scale up, water flow will reduce and the cell will overheat. This will damage the cell