Things were going swimmingly until I return from vacation to a slightly green pool. Pool store advised Zappit shock (73% calcium hypochlorite). The pool cleared up and was looking good. We cleaned filters, brushed, and cleaned up floats... The chlorine levels were high for about a week...in the 20's. My CYA was 80, but pool store said that was great. FC went down to what I think is the standard (3-5) and a few weeks later the algae starts appearing in the corners by the steps. I put in 8oz Zappit, the pool looks clear again. The pool store is telling me I should just put the zappit in the pool every week as part of my routine. After reading TFP, I'm not going to do it. I'm assuming I need to drain water, stop using pucks and go with liquid chlorine, bleach. How much water do I need to drain? The water table is very high here in Florida.