I believe that all the mosses that are being sold as aquatic mosses are actually mosses that occur naturally in very wet places or near bodies of water. The only moss that I would tend to call entirely aquatic is Fontinalis but even that may be found emersed. Your best bet is to try the one you like best and see how it performs. Not really into ferns and fungi.

(1) Create a Drainage Layer: You need to create a drainage Layer for your plants. If the plant's roots are left in standing water, then the roots may rot causing you problems. You can avoid this by using a layer of pea gravel or lava rock to create a drainage area for your plants. My drainage layer is between 1" to 1.5" deep. (2) Non-Toxic Substrate: The quality of the substrate you grow your plants in is very important! You do NOT want to get ordinary potting soil. Potting soil is filled with fertilizers and other additives that is great for plants, but can be harmful and sometimes fatal to your herp. I know a zoo keeper who uses "Edna's Best Potting Soil" to create vivariums for the zoo's herps. It can be difficult finding a retailer that sells it so please visit their website and type in your zip code to find a store near you that will carry their soil. Their website is http://www.ebstone.org/13_ednas.php. (3) Planting your Plants Do not simply stick your potted plant in the soil of your terrarium. Please remove the plant from your pot, and gently pull away as much soil as you can from the roots. Then rinse away the dirt from the roots in room temperature water. I prefer to treat my water by putting a few drops of de-chlorinator. If you don't have this, you can leave your water out in a sterile, non-metallic open container for about 24 hours. This will allow the chlorine and the fluoride to evaporate. (4) Fertilizer Do NOT use synthetic fertilizers! It can be fatal to your herp. If you're plant is in serious need of a pick me up, I would suggest using an organic fertilizer. (5) Don't ever use pesticides on your plants. (6) Maintenance I think this is the best part. You can shape your landscape by religiously trimming and training your plants. These geckos are nocturnal so I never see them during the day. I was sick of looking at what appeared to be an empty sad little terrarium so I created a vivarium that was aesthetically pleasing to the eye during the day, but still a great home for the geckos. I hope this helps! Here is a link that might be useful: The Apartment Garden

Wow this thread has been ages hahaha but anyway i just wanted to share cause i also want answers. Hopefully someone has a good theory about it. I have a closed terrarium/vivarium that i tried or shall i say experimented. It looks interesting cause I tried building it on a 3 liter glass container. I learned a little about creating it but i would say i dont have any background of biology or other sciences. So i piled the largest pebbles that could fit to the container's mouth that has a hole of an inch. The height of the pebbles from the very bottom is around an inch. Next to the pile is white sand that i dried up from a beach for half an inch, next to that are colorful beeds-mixed red and yellow to make it colorful if u look at it from the outside. Thats half an inch as well. Next is clay soil for half an inch, then pounded charcoal for half an inch. I heard that charcoal could clean the air or stabilize the atmosphere inside this but im not very sure of it. Hahaha Lastly, is a pile of loam soil which is ideal for the plants to grow. I planted moss which i got from the bark of a mango tree, a few fern, and few grass that only grow around 1/2 inch. Poured 5 tbsp of rainwater. Apparently its close and it can get enough sunlight to suntain its life. Its 11 months now and never been opened. It looks healthy cause the water does its cycle inside the bottle like the rain. And u know what? The interesting part is, i saw really tiny insect and a spider that came to life. BDW i didnt put anything in it. Do u think that was just normal or was it just part of the life cycle inside the bottle? Hahaha

It is a light diffuser and you will find it in the light section of a hardware store. It is used to creat a false bottom so that if you have a water feature then the water has somewhere to drain to without soaking your substrate and you have somewhere for the pump to sit, etc. You can accomplish the same thing sort of by just using something like gravel for a drainage layer under your substrate, but this really cuts down on the weight of a tank and that is good if you ever need to move it.