Trichlor (chlorine tablets)
Also called: chlorine tablets, pucks, trichlor tablets, stabilized chlorine.
Trichlor is the slow-dissolving chlorine tablet, about 90% strength, used in a floater or feeder. Every tablet also adds cyanuric acid (CYA) and is acidic, so over weeks CYA climbs while pH and total alkalinity fall.
Why it matters
Tablets are effortless, and that is the trap. Trichlor tablets are about 57% cyanuric acid by weight, and CYA does not evaporate or break down. It only accumulates. As CYA climbs you need more free chlorine for the same sanitizing power, until it feels like the chlorine stopped working. That is over-stabilization, sometimes called chlorine lock, and the only real fix is diluting with fresh water. Tablets are a fine tool used on purpose: covering a week away, or nudging CYA up early in the season. They are a poor everyday habit.
In numbers
Raising free chlorine by 10 ppm in 10,000 gallons takes about 18 oz of trichlor, slow-released in a feeder, and stabilizer comes along with every dose.