One of the toilet tanks in my house is leaking water slowly into the toilet bowl. A pic of the inside of the tank is attached. I believe a gasket is the culprit (the pink gasket pointed to by the red arrow in the pic), but I can't figure out how to remove all the black parts above that gasket to get to and replace the gasket. I tried pulling on and twisting the small part above the white ring to release the cylindrical part (which slides up and down) to which the gasket is secured but no success. Can someone please advise? Or, do I have to somehow unscrew the entire assembly (at the joint pointed to by the green arrow)?