Distilled water<>Deionized water<>Demineralised water
Deionized will be fine, but be aware that there is the potential for corrosion with mixed metals in the loop (unlikely these days) so you could use an anti-corrosion agent. Also, unlike distilled water, which is basically non-conductive, deionised and demineralised water are slightly conductive, so be (even more) careful of spilling them on live electrics. And there is likely to be live micro-organisms in your deionised water, so a Biocide is a good idea too eg. PTNuke.