Difficult to draw any correlation unless you had mixed an equivalent mix at the same time using tap and kept them in the same location as a control. The mould could have been due to higher ambient temperatures for example. Having said that, tap might be chlorinated which is a deterrent for a while. You could try adding a few drops of strong acid to your mix and/or keeping it in the fridge to deter mould/bacteria.
A few drops of Normal Hydrochloric Acid normally does the tricks but failing that you can use a food grade mould inhibitor called E202 Potassium Sorbate obtainable from here - http://www.the-home-brew-shop.co.uk/. Add 0.2g per 500ml of solution.
You shouldn't get mould with a commercial product. I would presume they put a mould inhibitor in. Normally in Europe they use methyl parabens but there's been loads of talk about possible health risks which are totally unfounded, but thats another story. Keep it cold and dark once opened or add a bit of potassium sorbate. Potassium sorbate does work best at around a pH of 6 so if your solution is >pH 7 it may not be too affective. I've waffled a bit, maybe just add a bit of hydrochloic acid and see if that works.