Bromine
Also called: bromine sanitizer, bromine pool.
Bromine is a halogen sanitizer chemically close to chlorine, with a softer feel and milder odor. There is no stabilizer for bromine the way CYA protects chlorine, and direct sunlight destroys roughly half of it in 60 to 90 minutes, so it belongs in indoor pools and hot tubs.
Why it matters
Bromine is a real sanitizer, not a gimmick. Outdoors it loses, because holding a level in an open-air pool is a losing, expensive fight against the sun. Indoors and in hot tubs there is no UV to burn it off, and it holds up better in hot, higher-pH water. One related trap: sodium bromide algae treatments leave bromide in the water until you dilute it out, and while it is there your chlorine keeps converting to bromine, which CYA does not protect from sunlight.