Answer: Algae starts as pollen and airborne organic matter. This matter falls into the pooul 24 hrs. a day. When it hits the water the chlorine kills it but contrary to popular belief the chlorine does not make that algae disappear. It is dead and must be physically removed from the water. This happens through your filter system. Water tends to move in a counter-clockwise rotation and if the algae is not captured by the filter then it will be pushed to the outer walls and in areas where there is poor circulation such as around steps, corners and the love seat areas. The algae will also tend to collect into rough areas of the plaster. All this dead algae needs is some help in getting filtered out such as brushing and/or vacuuming.