i dont think antifreeze is such a good idea ... it is too viscous and will slow your flow rate
in your car this doesnt matter cause the pump is driven by an engine but to a small electric pump this is more of a strain
the reason why antifreeze is used in cars is to prevent corrosion, raise boiling point and lower freezing point. A water cooling rig doesnt need the last 2 and the first can be accomplished by using distilled water
in my opinion pure distilled water is the best thing, water wetter might work but i have been running my rig with just distilled water for a year changing the water every 3 month or so and i havent noticed any corrosion.. infact i took the water block off 2 weeks ago and opened it and no corrosion at all