I should have stated naturally soft water. A water softener does an ion exchange with NaCl making MgCl/CaCl I think. These are salts which wouldn't be good for your radiator either. I think I read it adds up to 300 mg/litre which isn't much but salt is salt. Reverse osmosis removes it via a membrane so it would be ok as welll as distilled water. On purified water check the label for Ca, MG or total hardness and chlorides. I think it is tap water just run through a filter. Distilled water or deionized water is your best bet.