The worker delivering the chlorine, five teen lifeguards and six bathers suffered symptoms including shortness of breath, chest tightness, scratchy throat and wheezing.
By Thursday night, all but two patients had been released from hospital.
St. Catharines Fire imposed a "shelter in place" order covering some 60 homes in the area around the Lion Dunc Schooley pool.
People living in those spots were told to lock up after a toxic gas wafted out from a utility building next to the community pool.
The residents were told they could open their windows and doors and restart their air conditioners around 8 p.m.