Get a silver kill coil and distilled water to handle any biocide requirements. What is an anti-conductive additive? Short of using something like fluorinert I don't think there is a way to prevent the water from ionizing given the constant contact with copper.
I was posting from memory but I think I actually meant non conductive additive (as in it doesn't suddenly turn the water conductive) - I once filled up a loop and the pushfit fitting were dodgy - ended up soaking a 6800 Ultra + motherboard but they both survived! Can't remember exactly what it was, was blue if that helps
Btw a chiller to handle that sort of loading would be ridiculously expensive, the good thing about a water loop is that if the temps are too high at first try you can just keep adding radiators or more powerful fans till the cooling is sufficient. You can also use it for underfloor air heating with rigs like that
