I will try and ask around cause when we were first building this up, we did not find any garden stores who know about terrariums unfortunately. Until I change it, i was thinking of watering with bottled water for most of the time and then some tap water every now and then (since you suggested that a little bit calcium in the water might be good for coco fibre). Do you agree? Also, we do not have the same problems on plants which are in pots (but as you mentioned, these have drainage so that could be the reason why). Thanks once again
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.
Thanks once again for your help. We have infact reduced the frequency of watering the plants (used to water the foliage plants every coupe of days and the cactus once per week) but now we are first checking (manually) if the soil is still wet before watering the foliage plants. I tried taking a clearer picture but unfortunately it's not easy cause to take a clearer photo, i would need to take the lid off and if I take the lid off, there is not enough light in the hallway (even with lights switched on) for the white stuff to show properly on the photo. I will take your advice Thanks a lot all of you for your help!!
