Are the lines on the expansion tank supposed to correlate to cold or hot temps? Cause cold my coolant is below the low line.
Distilled water is what you want to use in the radiator. Distilling removes more contaminants from the water.Those are "Full" and "Low" markings for cold condition. When hot, the level is higher. The system is pressurized to prevent boiling up to 120°C, so never open when hot or warm. Fill to the "Full" with deionized water (you likely have sufficient antifreeze unless you live in a very cold place).
If you overfill, the excess will get spat out that little black hose (harmless but a bit messy).