Actually, you have another option: don't use any.
If your rig only has copper and brass touching the coolant, you will be ok. Cathar reports using tap water (only) for 7 months, with only a slight yellow tinge to it. Copper and brass setup.
I've also got compiled data of 4 other users here, and one more over at OC, that use no additive, and other than some biological stuff, one user reports a white powder deposits, with Alu in his loop.
2 of those users are here, without Alu in their loops, and only reported white deposit on their tubes, as did many others with additives.
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