I have done the BBB switch and plan to add the "closing" chemicals to my pool before my PS comes to do the actual closing for me. I am wondering about the Polyquat. I currently have lots of unused PS chemicals, one of which is Biogard Algae Back Up. According to the label, it is a broad spectrum algae preventative, consisting of Alky (C 14, 95%; C12, 3%; C16, 2%) dimenthyl benzyl ammonium chloride - 30% and Alkyl (C12, 61%; C14, 23%, C16, 11%; C8 & C10, 2.5%; C18, 2.5%) dimethyl benzyl ammonium Chloride - 10%. I am not a chemist, but hopefully someone who has that wonderful knowledge can tell me if it can be used instead of the polyquat. If it is NOT as good, I will get the Polyquat. My pool is in wonderful shape and I don't want to mess it up now. I use my heater in the fall and don't close it until mid October. Also, if I have been reading the multiple threads about pool closings correctly, there are different kinds of Polyquat? a 60, 30, etc? Which would I use if my Back Up is not sufficient? KEEP UP THE GOOD WORK!!!!!!