I would change the water in the bottom of the tank for a couple reasons. 1) It will look better if the water doesn't have gunk growing in it. 2) Minerals from your water will gradually build up in the water in the terrarium. The build up will be slowest if your using distilled water and fastest if you are using tap water. The mineral build up won't bother your plants as long as they aren't sitting in the water, but they will eventually begin to form deposits along the sides of the terrarium.