Don't know about your test results, but your chlorine demand may not be because of the old tabs--you don't post readings for FC and CC, or for CYA. I suspect that if you are running your trichlor feeder wide open, then your CYA may be very high, high enough that the tabs can't provide adequate chlorine. If you'll post current numbers for FC, CC, and CYA, it would help us figure it out...

Janet