What water to add to the heater water tank?

Of course, add coolant. Generally, coolant needs to be mixed with half coolant and half water, and distilled water needs to be used, because the minerals in tap water will change the properties of the coolant, and It will make it unable to operate properly, so never add tap water. The adjusted coolant can be added from the water filling port of the water tank. When the coolant reaches above the lowest level of the water tank and below the highest level, it will be fine.

I hope my answer can help you.