Hi, just joined TFP today. I live in Houston and have been struggling with green algae in the pool. Leslie's pool supply wants to sell me ammonium sulfate but I've read that this can do more harm than good in the long run. I like the idea of SLAM but I have one question. Using the calculator for a CYA reading of 50, I get a result of 20. Is this the desired FC shock level or the amount of liquid chlorine in fl oz to add to the pool? I assume it is the desired shock level. Seems VERY high to me. How much liquid chlorine would I need with 13000 galloons pool water to raise the FC level to 20 ppm?