overflowing coolant

[walian at att.net]

Hello all,

 Can replacing the coolant with straight water make the
expansion tank overflow? I have just put the timing
belt and refilled the radiator with water. I drove to
work this morning(20 min) and noticed that the car was
low on coolant(water) a few hours later when I went to
run an errand the emergency light came on and the
expansion tank was completely empty. I filled it to the
brim with water and drove home, no problems. I just
came back from a store and the radiator expansion tank
is now leaking water. Can this be due to straight water
or a bad thermostat which I just replaced?

Thanks

Frank