The bottle of Bleach is an item obtainable in Silent Hill 3.
It is found on the counter of the first floor women's washroom in the Otherworld version of Central Square Shopping Center. When combined with the detergent, it creates a vapor that acts as an insecticide and clears the 2F hallway of the moths blocking Heather Mason's way. The vapor is highly toxic, so before doing this action, Heather must turn off the fans by pulling the lever that is located at the end of the previous hall.
The bleach will not appear if the puzzle difficulty is set to Easy. Moths do not block Heather's path so the bleach is unnecessary.