These pink stains are usually caused by biofilms and are not harmful. The pink growth is not from the water, but from bacteria or other microorganisms in the air that thrive in damp areas. It is mostly observed in the area between water and air, in locations that stay moist. Biofilms can also be found in shades of yellow or brown, orange, or red. Because they thrive in the presence of water and small amounts of nutrients, biofilms are found virtually everywhere there is water, including sinks, toilets, shower curtains, humidifiers, ice makers, and water purification systems. Scrubbing and chlorine bleach are the best solutions to eliminate the occurrence of the pink residue. An easy way to do this is to stir three to five tablespoons of bleach to the toilet tank, flush the toilet to allow the bowl to be disinfected, and add another dose of bleach to the tank as it is refilling. By keeping bathtubs and sinks wiped down and dry, the formation of pink residue can be avoided.