I have a 10 year old Salt Water Pool (white plaster). Came back from a 2 week vacation and the water looked great... clear and Free / Combined Chlorine were 3 PPM. One week later it went all to ****. (We have a safety cover, so I don't always see things happening). The water was very cloudy and I had no chlorine. After a few hours, I realized that I had DE in the pool. (The fitting on the top of the filter manifold was blown apart, but I was able to find a dealer open 30 miles away (it was late Sunday, so I'll take what I can get). After cleaning the filter grids, I realized that an O-ring was cracked and wrapped with teflon tape... nice. (a surprise from the guys that open/close the pool). That's now fixed, so the DE should either be in the filter or the lawn, but not the pool.
So for the new problem... the pool store (closer to me) did a demand test and said that I need 52 lbs of shock. Every time I shock to their level, it takes me a long time to get the Free / Combined Chlorine down to a readable level (even with 50-50 dilution). So far, I've put about 27 lbs in and the Free Chlorine is beyond readable... should I add the rest or see what happens over night? The water looked good before shocking, but it's cloudy now. (The pump pressure is rising, so I assume I'm getting rid of Combined Chlorine.
I have a new Blue Essence Chlorine Generator, so I don't think I have a problem there (and the chlorine is too high to try to crack open the fitting and test the water as it exits the cell).