In the past few weeks, I've seen a handful of pools that won't hold chlorine at all. Usually when I see this, it is a pool with phosphates, and we either treat phosphates and/or shock the pool. Lately several pools have done this without a phosphate problem. Shocking them doesn't seem to solve the problem like I would expect. Usually 4lbs of trichlor will bring a pool back even after it has gone green and cloudy. These pools are usually still clear, a few have been slightly cloudy, or have had small amounts of algae growing on the liner. What seems confusing is that I've seen a few get quite cloudy and come right back with 4 lbs of trichlor, but others will be crystal clear, and just test low for FC and don't seem to be phased by it. I would think the cloudy pool has a bigger algae problem and would eat up chlorine faster. So why am I having trouble getting these pools to respond. In all of these pools, chemistry is all where it should be except for Chlorine levels. Most of them are using SWGs. Most of my pools are running somewhere around 50% this time of the year. The pools in question typically start showing lower than usual readings (around 1 or 2 ppm FC, sometimes lower) and even after turning the SWG up to 60 or 70% for a week, we don't see it rise.
This all sounds like algae trying to bloom, but why does it take so much to stop it? One of these pools had been shocked several different times, and finally started holding chlorine once I put in 12lbs of trichlor. We tried 6lbs a few days before, and the next day there was no FC in the pool. Any thoughts? Is it normal to have to shock this hard? Am I wrong to feel like 12lbs of trichlor is excessive? Or am I on the right track with hitting these pools hard with trichlor?