Your tabs are almost certainly trichlor and they have cya (stabilizer) in them. All pools need some cya in them to keep the chlorine from being lost to the sun too quickly. However, when your cya level gets too high, you have to continue to run higher and higher chlorine levels to keep algae at bay. A lot of people don't know this and they use those pucks for a very long time, unknowingly allowing their cya level to get way too high.

It would probably be helpful for you to read a lot of the "stickies" at the top of most of the "Pool Water: Treatment, Testing and Problems" section of the forum. A lot of the pool care chemistry basics are explained there. You might want to start a thread in one of those sections and post a current set of water testing results for someone here to look at and maybe head of a potential problem that you may not be aware may be brewing.