Tri-Chlor is the standard 3" tabs that people put in automatic chlorinators or floaters, sold in every pool store.

Tri-Chlor adds chlorine, but also adds acid and stabilizer to your water. Sometimes people put the tab in the skimmer, which CAN be a problem when the pump is off as the erosion continues and concentrates chlorine in the lines into the filter or in the filter itself. But I don't believe it is USUALLY a problem.