Well once it is daylight, try to see where the water is coming out. You say the coolant tank won't hold water? It's right there so where is the water coming out of? Same thing for the radiator. You are adding water to the radiator neck and you would see if it was leaking out somewhere. The more serious issue is whatever is going on near the header. Is is just an exhaust leak or do you have a blown head gasket? If it was a blown head gasket that would explain why you keep losing water. Check your oil. If it is turning milky, that means coolant/water is mixing in the oil and you probably have a blown head gasket. White smoke coming out of the exhaust also means coolant/water is getting into the combustion chambers = likely blown head gasket.