Some bottled water is tap water anyway, here at least. Even bottled natural spring water still has calcium in it (it says how much on the labels here). I'd just go ahead and use the tap water. But with a more or less enclosed terrarium you shouldn't need to be watering very often at all, I would have thought. Maybe get a moisture meter/indicator and see what the level is before watering. You mentioned earlier giving them some water every day, which I wouldn't have thought necessary. If the lights are incandescent and giving off too much heat then can you switch to fluorescent (or maybe LED in an appropriate spectrum)? Or put the plants nearer the window? Perhaps you have a conflict with the amount of light needed since you have a cactus (high light and low moisture) as well as foliage plants (lower light and higher moisture)?
You could try the houseplants forum here for suggestions as to what other people are using as substrate, and if you type terrarium into the search box top left there are a couple of article results that might give you some ideas what to use as a growing medium. I looked at a few articles and some suggest a peat-based ordinary potting mix with caveats that perlite etc. should be taken out if there are reptiles in the terrarium.
