Victoria,
Australia 
Alright, so I recently was given a terrarium as a gift and I absolutely love it! I'm not sure what plants are in it, but I'm pretty sure it contains some kind of moss. (I attached a photo)
When I received it, the only instructions I was given was to spray it once a day. I did some research and many people said that a spray a day was too much and that it only needs to be sprayed once every couple of weeks. I decided to stick with the instructions I was told and would reduce the watering amount if I saw any build up of gunk or rotting. So over the past couple of weeks I have been spraying it once a day. It receives good sunlight through a window, but the sun isn't harsh because winter is only just ending. I have notice though that the leaves have started to turn brown as if they are dying. My immediate response was to start watering them more, because I thought that they were drying out, however when I touched them they were moist. There was no build up of water but some of the leaves looked a bit gunky. I did some research and discovered that it is most likely due to the minerals in the water building up and slowly killing the moss. I didn't know what to do and so I stopped watering my moss to prevent any further damage. Now my moss is drier and still brown.
Why is my terrarium dying? Is it because of the minerals in the water and if so, is there anyway I can save it? Will using filtered water save it or is it too late?
Please help! I hate seeing it dying! :(
29 Aug, 2015
Answers
Get it out of direct sun - no matter how weak the sun's rays, they will be magnified by the glass container, and moss particularly will hate that.
What you've got in there are two different varieties of Hypoestes (commonly known as Polka dot plant, and a good terrarium subject) and sphagnum moss. Now, the Hypoestes likes a good, bright light, and a bit of sun doesn't go amiss, BUT, the moss will not like any sun when its sitting in a glass container. So the best position for your terrarium is in a brightly lit (by daylight) sunless position. The sunlight its received so far is likely why the moss has browned - but also note that, if the moss fronds come into contact with the glass, browning occurs then too.
Instructions for watering moss in terrariums are every 1-2 weeks, but that assumes you're actually watering and not just misting, so I imagine every other day will be fine. If you notice the Hypoestes wilting, then they're not getting enough moisture, so see how it goes. If you're not sure how dry things are, you at least have a fishbowl type of terrarium which means you can get your hand in there and poke around to see how moist (or not) the soil and stones are. Tapwater's fine, but if you live in a hard water area, you might consider boiling the water and leaving it to cool before using to get rid of the lime content, or using distilled water instead - there's a risk you'll end up with lime/salt deposits on the glass otherwise.
29 Aug, 2015
Fittonia, actually, Bamboo, still a good terrarium subject. Unless the terrarium has a lid, or the room humidity is very high, I would mist the moss every two days, with distilled or reverse osmosis water--charcoal filters won't help enough.
Sorry, Bamboo, but boiling won't help hard or salty water, in fact, it will make it worse. Only distilling, reverse osmosis, or de-ionizing actually removes the culprit calcium and sodium ions.
29 Aug, 2015
Ah, I was thinking purely of limescale - which will be removed by boiling. I know that cos its all over the inside of my kettle!
And you're right about the Fittonia for the pinkish one, but I'm still not convinced the red leaved one isn't Hypoestes phyllostachys, there are loads of varieties that look very similar.... though having looked some more, there is a variety of Fittonia called F. verschaffeltii which is pretty close... I'm guessing it don't matter too much, the care will be similar.
30 Aug, 2015
Fittonias, in my experience, are more sensitive to over watering, though neither could be called drought tolerant.
30 Aug, 2015
Are you using tap water?
The coleus look healthy enough so it is just about getting the water right for the sphagnum(?) moss - which will prefer damper conditions. The coleus wants moist but not soggy, while moss will prefer soggy!
When you say 'spray' are you drenching the plants or just giving a light misting? The moss will benefit from daily misting but the coleus won't, so be careful where you are directing the water. Originally, a terrarium would be sealed so that it only needed watering once a year - when plants were pruned - what you have really is an attractive planted bowl with plants that have different requirements.
If you have an aquarium store nearby try getting some RO water. You could also try collecting rain water. Apologies for my ignorance on the rainfall subject but, if you are in an urban area and you get a heavy downpour then start collecting rainwater about 15-20 minutes into the downpour - fish keepers who use rainwater in their tanks do this, it means there is less chance of air pollutants in the water. If you are lucky enough to live in an area without air pollution then there's no need to wait.
You could also try misting with bottled (still) drinking water which will have some minerals but won't have the chemicals used to keep tap water clean.
29 Aug, 2015