They are introduced into the pool by airborne spores, from make-up water, from covers which have been dragged over grass and moss and from vegitable matter. Onset is quite common after thunder storms, especially if the water temperature is high. Algae take up dissolved carbon dioxide and therefore cause the pH to rise rapidly. They can be the clinging type and colonise the pool surfaces, or float suspended in the water. Normally not a problem if chlorine levels are maintained.
Elemental bromine is a reddish brown colour and is liquid at room temperature.
The hypochlorous acid serves to regenerate the spent bromine. It is dosed into the pool by means of a circulatory feeder. A dry chlorine donor available in the form of granules or tablets made by the absorbtion of chlorine in lime. It tends to raise the pH, but this and the presence of calcium is beneficial for pools in soft water areas. Too much calcium means cloudiness and scaling up, too little could lead to the water satisfying its appetite for calcium by taking it from your grouting.