Most of the time a reaction had occurred when adding the content of an old container to a new one or when scooping from the latter. An acrid gas evolved and, frequently, someone had to be taken to hospital for medical attention. On most occasions, the premises had to be evacuated.
Most swimming pools recirculate their water in a closed loop.
New water is added from time to time but refilling is rather unusual. A subtle balance must be obtained between many different chemical processes to maintain the quality of the water and the required health standards. Several biological and chemical phenomena will occur and must be controlled by adding different chemicals. Algae may readily form from airborne spores. Bacteria, brought in by the swimmers themselves may prosper.