Why does my water leave this pink film?
What you're most likely looking at is a bacteria called Serratia Marcescens.  

    

According to Wikipedia it is commonly found growing in bathrooms (especially on tile grout, shower corners, toilet water line, and basin) as well as many other places.  

From that source (and others), the best way to get rid of it is regular cleaning by soaking &amp; scrubbing with bleach-based disinfectants.