You might be thinking of certain types of pool chlorine tablets that include cyanuric acid. Really a terrible way to treat a pool, honestly, it builds up CYA i

You might be thinking of certain types of pool chlorine tablets that include cyanuric acid. Really a terrible way to treat a pool, honestly, it builds up CYA in the water and eventually you can't get enough active chlorine to safely disinfect. And the only realistic way to reduce CYA is drain the pool partly or completely and refill. Properly maintained pool water only needs basic chemicals, and pure sodium hypochlorite (unless you're going to use a chlorine generator, in which case you'll end up buying a few hundred pounds of salt when you fill the pool).