FC is "free chlorine" - which is what kills, CC is "combine chlorine" or chlorine that is used up (causes chlorine smell and burns eyes) and TC is "total chlorine" or just FC + CC.

You want your FC at 3-6ppm and your CC under .5. When your CC is above .5 - that is when you need to shock it.

What are you testing your Chlorine with? If you are over a certain amount some tests actually bleach out and it shows no chlorine. The experts here can explain which ones are the best (I believe the DPD-FAS).