If you remove ions from water (deionized water), the water will pull metal ions from your loop... That is not good. Use distilled instead of deionized.
Distilled is more than good enough for any water loop, the most important factor isn't water purity, it's algae content (don't use tap water), loop bi-metal setup (is it a battery?), water mineral content (don't use tap water), anti corrosive additives (don't use just distilled water), water freezing point (use anti-freeze) otherwise your loop will burst and leak at freezing temps.
If Water Block/Radiator is Copper/Aluminum, bad news (it will corrode in a few years, battery effect), if it's Tap Water or has Algae in it, bad news, it will clog in a few years (algae bloom clogs WB fins), if it's impure (not distilled or tap water), bad news it will clog (minerals will clog WB fins), if it doesn't have anti corrosive agents in it, bad news it will corrode (it might corrode, clog in a few years via WB fins dissolving).
Copper-Brass is good for Waterblock-Radiator setup. Copper-Copper even better, but most importantly, use distilled water with anti freeze and anti corrosive additive and absolutely don't use de-ionized or tap water, don't use a bi-metal setup like Aluminum-Copper, don't skip antifreeze if you live in temperate climate (temps go below freezing).