I just bought an EK EN nickel 580 water block, and the sheet that comes with it mentions fluid. I shall quote an excerpt from it:
"Certain biocide additives such as copper sulphate, silver coils and even pure distilled water only may, when not used with corrosion inhibiting coolant, cause irreparable damage to the nickel plating due to accelerated galvanic corrosion"
Hope that's been of some use
As for the block, I'll echo everyone else's comments. Go with what looks nice/is a good deal. I went for the block I did because I wasn't too bothered about the block cover material, and it was brand new and going cheap on the member's market.