What is the pink buildup in my toilet?

The bacteria will grow in any moist location where phosphorous containing-materials or fatty substances accumulate. The bacterial colony, can appear to be fuzzy or slimy. The color range is affected by room temperatures. Sources of these substances include soap residues in bathing areas, feces in toilets, and soap and food residues in pet water dishes. Serratia can also grow in tap water in locations such as toilets in guest bathrooms where the water is left standing long enough for the chlorine residual disinfectant to dissipate. Serratia marsescens is not known to cause any waterborne diseases.

Periodic and thorough cleaning of the surfaces where the pink slime occurs, followed by disinfection with chlorine bleach appear to be the best way to control it. Scrub the surfaces where phosphorus and fatty substances, or the bacteria accumulate with a brush and a household cleanser. Then disinfect the surfaces where the slime has formed with a strong chlorine bleach solution. Leave the disinfectant solution on the affected surfaces(s) for 10-20 minutes before thoroughly rinsing it away with clean water.

Note: Bleach should not be left in the toilet tank for prolonged periods; it will damage the rubber valves and seals inside.