Always fill your radiator with correctly diluted antifreeze that has aluminium corrosion inhibitors. Don't fill it with tap water as this causes deposits in the coolant channels and corrosion.

Regularly check the water levels by removing the main radiator filler cap. Do not rely on the level in the overflow catchment tank as an indicator of the level of coolant in the radiators - this is just a catchment tank for overflow should the engine get hot.