According to Dr Dunn, calcium, iron, copper and nickel can cause DOS - all of these can be in tap water to a small degree.
If you buy purified water make sure it is below 6µS (µS means micro-siemens) conductivity, otherwise you could also use de-ionized or even better, distilled.
In England, its v v hard to get distilled water so I generally get deionized from a petrol station or purified water from a chemist so long as its below 6µS conductivity.