With a CYA of 100, you're going to need to shock (super-chlorinate) up to the 20-25ppm level and hold it there until you no longer have any combined chlorine (CC). I have a feeling you've been yo-yo'ing w/ your shocking. One gallon of 6% bleach will raise the chlorine 6ppm per 10,000 gallons. So, on a 15K gallon pool, one gallon should raise you 4.5ppm. You'll need about 5 gallons of bleach in your pool to initally raise it up to 20-25ppm level. Make sure you're running the filter 24/7 during this process. Backwash as needed as algae can gunk up the filter.
Also, your pH is too low and may be lower than 6.8. Most test kits don't test below this level, so I'd do this first. You'll need to bring that up to 7.2 at least.
Stop using the chlorinating pucks. They drive up CYA and drive down pH. Begin using bleach only for chlorinating. You'll find a lot of information in the chlorinating section http://www.poolforum.com/pf2/forumdisplay.php?f=132
as well as tons of tips to get you started http://www.poolsolutions.com/tips/10things.html
Good luck.
CaryB
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