I am making a terrarium and almost everywhere (on the web) it says to add a layer of activated charcoal between the soil and drainage rocks. I plan on creating a sort of 'meadows' look by making small hills and planting it with moss then adding a few trees. Thus there will be a LOT of moss in the terrarium. Is the layer of charcoal still needed if I have so much moss? I read that moss can have some of the same effects as charcoal such as helping reduce mold and mildew but also read that it can do the exact opposite. Would this then mean that when having so much the need for a layer of activated charcoal is higher?
Some useful info
The terrarium is closed and does not have any drainage.
I will be watering it with clean pond water, so no chemicals in the water.