Use the coolant type specified in your owner's manual which is a HOAT. Or temporarily you can always just top it off with distilled water which your local grocery store carries by the gallon. Not filtered water, use only distilled water which has none of the dissolved mineral content present in most drinking water. Most tap water is high in dissolved mineral content which when in a radiator, is bad news.
When you completely flush & refill your cooling system, a HOAT coolant like Zerex G-05 is a good way to go. Or you can just add some Zerex G-05 premixed HOAT coolant now which better parts stores like NAPA carries. My local Autozone stores don't carry anything that good.