A universal canister flush valve having a valve body configured to be fixed relative to a toilet tank and having a hollow wall defining an internal flow passage; a guide post coupled to and extending away from the valve body; a float fitted about and configured to slide relative to the guide post between a closed position and an open position, the float having an open top; and an extender that selectively couples to the open top in a first position, in which a first end of the extender is received in and coupled to the open top, and in a second position, in which a second end of the extender is received in and coupled to the open top, wherein the extender and float define a first overflow height in the first position and define a second overflow height in the second position.