One time, I dropped a small, 1" chlorine tab from our pool into the tank of our toilet. And holy fuck, let me tell you, that shit had our toilet looking far cleaner than it could ever get with regular cleanings, with zero effort. I eventually took it out because apparently it can cause the seals and whatnot to degrade quickly, and eventually you'll come home to just water pouring out of your tank.
But god damn, did that shit make my toilet look brand new. I since tried toilet appropriate tabs, and they didn't even come close.