The red or pink pigmented material that you will find in any moist environment in your house is actually an airborne bacterium called Serratia Marcescens, it is not from the drinking water itself. The pink ring can develop at the water line in the toilet, around drains, in the tub/shower area, in bathroom drinking cups, and even dog bowls. The airborne bacteria thrive in moist environments, which is why it is commonly found in bathrooms. This bacterium can come from many natural places and can be greatly increased in homes where the chlorine residual has been removed due to activated carbon filters, other water treatment devices, or where the water has sat for a period of time lowering the disinfectant residual. These non-pathogenic bacteria cannot survive in chlorinated drinking water. Commercial cleansers including bleach are useful in removing the bacterium but regular cleaning will be needed to reduce the occurrence.