How do you treat biofilm in a pool?

1. If the water is cloudy, a flocculent may be used. Do not use a flocculent if you cannot vacuum to waste. 
2. Add chlorine. Note that it may take several times the normal amount of chlorine in order to oxidize the protective layer and reach the trillions of organisms living inside. 
3. Brush. If the biofilm (or algae) has adhered to the surface of the pool, brushing will help break through the protective layer and release the microbes into the body of water where they will come in contact with the sanitizer
4. Add algicide. If you are dealing with an algae bloom, use an EPA-registered algicide per label directions. 
5. Clean the filter. It is very important to chemically clean the filter. These organisms can grow inside the filter bed and re-infect the pool.