Adjusting Betta fish to new water.?

You see I recently bought a male Betta fish. I have him in spring water, but since his tank does not have a water filter, I need to clean the tank once a week. I want to get him adjusted to tap water since it is cheaper. the tank is a small 1.5 gallon, and is not aerated. i also have 1/2 gallon of spring water left. If I mix that w/ the tap, and then add another half gallon of tap, will it be okay? i bought it in those 1 gallon milk jugs. can i put the water in there and let it sit for two days so the chemicals can break down? also, would i leave them with the caps on or off? please help!