Manufacturers of Al. rads. recommend distilled water because alot of tap water contains Chlorine and that is VERY corrosive. If you don't use anti-freeze, you need to use an anti-corrosive additive. A sacrificial
zinc anode in place of the drain plug is a great idea. Pick one up at any boat supply place. Another good idea is to make sure your rad. is grounded so there is no stray current in your cooling system.