What are the forms of chlorine used to treat public swimming pool?

Almost any form may be used on outdoor pools. Some areas prohibit or restrict the use of stabilized forms of chlorine, like trichlor or dichlor, on indoor pools.

Chlorine gas used to be the most common method form used on public pools. But, because of restrictions, it has become rare, and sodium hypochlorite (bleach), calcium hypochlorite (HTH, etc) and trichloro-isocyanuric acid (trichlor) are all more commonly used.