Crikey, just how hard is your water that your cham has limescale? ;-)
Bottled water is a bit open ended as many bottled waters are 'mineral waters' containing all sorts. Good for us (in moderation) but not necessarily for chams.
RO would be a better bet, either bought, from local aquarium suppliers or anywhere else you know. You can buy your own 3 or 4 stage RO filters which are easy enough to set up. For misters, foggers etc., RO is usually the prefered choice for most folk.