Is pink pool water caused by algae or bacteria, and how do I fix it?

Pink pool water is caused by Serratia marcescens bacteria, not algae. Super-chlorinate (shock) the pool to 10–15 ppm free chlorine, brush all surfaces, and clean or replace the filter media. Run the pump 24 hours, keep pH at 7.2–7.4, and maintain steady chlorine to eliminate the biofilm.