Quote Originally Posted by AnnaK View Post
It's my understanding that cyanuric acid is a 'weak' acid.
True. And, dumping cyanuric acid on a liner probably won't do any damage. John A. Wojtowicz reports that the pH of a saturated solution of CYA is 4.5 - 5.0 -- not good for a liner, but not panic time, either.

However, various sources (for example, here) report that the pH of a saturated solution of tri-chlor is around 3.0, which is definitely worse. But, it's worse than that. The zone under a trichlor tablet not only contains dissolved trichlor (pH 3), but the trichlor releases chlorine, which at that pH forms a mixture of dissolved chlorine with other things. And chlorine in water at pH 3.0 is VERY corrosive.