Originally Posted by Kingtal0n

Use distilled water in anything you care about

coffee makers, steam cleaners, auto-claves, and engine cooling systems.

A major goal of (engine/reactor) cooling and use of water in devices is to minimize the ionic/mineral content of water and keep it low by continuously flushing over the years with fresh distilled water (and necessary additives). Electrolysis corrodes engine metals away and occurs when electrical conduction takes places which can only happen when there are ions present. Water purity is often measured in terms of its conduction. So the goal is to minimize conduction/ions which means starting out with as few as possible.

The reason some coolants say "can be mixed or used with tap water" is because they contain chelaters and molecular shells which scavenge some unwanted entities from tap water to render them harmless. But that is sort of like drinking dirty pond water and then expecting your immune system to sort the mess out; Instead, just drink the pure water to begin with

When you buy 50/50 premixed coolants that 50% of water is distilled water

I recommend 90% distilled and 5-15% green antifreeze for Florida cooling systems if the cooling system will be shut for 5~ years or more. Otherwise 100% distilled is great because you can spill it anywhere.