I bought a 50lbs pail powdered pool shock last season. Good thing I use nearly 50% of it... it seems like now it just sinks down and does not do anything at all even to shock the pool now. I should have bought individual packs since they are all 'sealed' . Never buy a pail... with the winter and not being air tight... it lost its 'cleaning' power'. Any suggestions of good 1lb bags of good quality shock and yet cheap? Amazon or intheSwim or chlorox ones?