Distilled water is a must.
Your choice of water wetter or anti-freeze is pretty much up to you.
Water Wetter is supposed to allow the water to move better through the system, but it doesn't actually improve the water's ability to absorb heat, However, it appears to coat everything with a thin layer of smegma (tm), and could eventuallt make your crystal-clear tubing semi-opaque and kinda brown looking.
Anti-freeze raises the boiling point, and lowers the freezing point of the water, but I don't think it improves the water's ability to absorb heat. It's also generally green in color (I don't recall seeing any other colors, but it's been over 15 years since I did anything mehchanical on a car that was more difficult than un-threading the fuel filler cap.
Either one will inhibit algea growth, so that's a good thing.