I recently bought an older house, and the toilet tank is leaking. In exploring the issue and googling around for toilet repair, I saw no mention of one of the parts that is in my toilet tank. Within the tank, there is a smaller plastic container that surrounds the flush valve. It seems to restrict the amount of water that ends up being used per flush, so perhaps it was added to reduce water consumption? What is this thing, and how does its presence affect my approach toward replacing the different pieces of the toilet?