Distilled or deionized? Deionized doesn’t conduct electricity. Also, don’t mix cooling plate materials. Use all copper or all aluminum. One of them will cause the other to corrode if sharing water.

Generally, it's good to have biocides and corrosion inhibitors so the loop doesn't corrode or develop biofilms. I only need about a liter per year and would rather not have to keep separate bottles of biocide and inhibitor. And I'd need to find distilled water locally. For about $12, a premix is fine.