Firefighters said chlorine spilled into the lazy river at Club Westside, a family sports fitness club owned by businessman "Mattress Mack," just before 5 p.m.
Authorities initially said seven children and three adults were taken to the hospital after possibly ingesting chlorine. In an update, HFD said 12 people were hospitalized.
Chief Samuel Peña tweeted that the spill was under control at about 8:25 p.m., applauding firefighter crews for quickly assisting club members. He said an over-chlorination of the pool caused the incident.