Quick Answer
Chlorine is a single-element disinfectant used widely; chloramine is a longer-lasting chlorine-ammonia compound some utilities prefer. They behave differently in the shower and are reduced by different filter media — check your water-quality report to know which you have.
Problem
Most people assume 'chlorine' covers everything. If your utility uses chloramine and your filter only handles free chlorine, it will not do the job.
Product Recommendation
ZOOSHEN offers systems with different media. For chloramine areas, choose a system with a vitamin C stage; for chlorine, KDF/carbon systems work well. See /technical/chloramine and /technical/vitamin-c.