While it's still warm I wouldn't replace low coolant with antifreeze. Just antifreeze doesn't provide any real cooling from the radiator, just protects the water from freezing during the cold months. Next time use straight water, and not from the hose, purified water, so the heavy metals and other chemicals are removed. This will help keep from getting rust or scale deposits from building up inside your radiator. If you have a leak add 50/50 mix, if it's from evaporation, replace with just water.