Quick Answer
Calcium sulfite is a filtration medium used to remove chlorine and is notable for working well in hot water. It is less common than KDF or carbon and is usually used as one stage in a multi-stage filter.
Problem
Calcium sulfite appears on some multi-stage filter packaging without context, so shoppers cannot place it among the better-known media.
Product Recommendation
Some ZOOSHEN multi-stage configurations include calcium sulfite as a supporting stage. Check the product specification for validated media after final validation.