These are simple tiny plants or seaweed that grow and multiply if nutrients are present and chlorine or sanitizer levels are low. Most algae are invisible except in millions. In sunlight they convert carbon dioxide into oxygen by photosynthesis. They are larger than bacteria and viruses but generally smaller than 5 microns and vary in colour from green to blue, brown and red. They can be seen inside water filter housings and on water filter cartridges, often giving a slimy feel.