I'm buying 10.5% Hypo from Leslie's and I'm in the middle of my first SLAM. SLAM is working great, but how long can the 2.5 gallons last before losing it's potency? Second question, maybe not for this thread, but how do I know how much chlorine to add during the SLAM? If I need to get to 15 ppm and I'm at 12, how much to add to get to 15? Is there a formula or do I just add a couple of cups and re-test? I'm using a TF-100 test kit.