I have a 04 civic. It keeps on leaking coolant from the overflow tank, but never overheats. I replaced the thermostat, radiator cap, cooling fan switch, radiator. Bad head gasket?

Classic head gasket failure. This often happens on Hondas by not following the Owner's Manual on how to properly drain and refill the coolant, to prevent air from forming in the system. Often, you must set the temperature slide on hot, turn the ignition switch to ON, and then turn it to OFF. This opens the heater control valve, on some models, and prevents air pockets from forming. Have the head inspected and machined, if necessary, before just replacing the head gasket.