Quick Answer
Soap scum forms when the calcium and magnesium in hard water react with soap to create a sticky, insoluble residue (a fatty-acid salt). It is why hard water 'does not lather well' and leaves film on skin and surfaces.
Problem
Soap scum on tiles, glass, and skin is one of the most visible signs of hard water, and people often blame the soap or cleaner.
Product Recommendation
Soap scum is a hardness issue, not something a shower filter solves. If your main concern is the shower-water experience with chlorine/chloramine, ZOOSHEN filtration helps there. See /water-science/water-hardness-units.