I'm growing some neps in my grow chamber. They were all cuttings and are all growing pitchers like crazy now. This is how I have it setup.
It is pretty much sealed, surrounded by mylar on every side. I use a mix of 4ft warm/deluxe t8 bulbs. I have a total of 4, 4ft bulbs on this grow shelf. I also have 2 80mm CPU fans, one on each side of the shelf blowing around in there.
I'm not sure how you are growing your Neps, are they planted in the terranium or are they in their pots? I have mine in their pots. Also what kind of medium are you using to grow them? I'm using 50/50 LFS/perlite.
Next, I bought a rubbermaid storage tub thingy, and filled the bottom of the tub with about 1 1/2 inches of pea gravel. I then water each pot once a week, and fill the tub with water until it's about 2inches or so up their 6inch pots. It takes about a week for it to all evaporate. I let the tub completely dry out for a day or so before I water them again. They seem to like this.
But as for tips, since you are using a 20 gallon tank, you can skip buying the plastic tub and just fill the bottom of your tank with a few inches of pea-gravel and top it with some LFS if you have any. Put your Nep pots on top of that and then water it and fill up your tank so that around 2-3inches of your nep pot is sitting in water. Then let it all evaporate and dry out, then refill it. They seem to really like this kind of watering cycle from my experience. As long as you are using a LFS/perlite or alternative light and airy medium you will not have any root rot problems. I would not recommend sitting your pots in water though if you are using pete. Also line the glass with mylar to get more light to them. Leave the top of your tank uncovered, I think that would be too much humidity.
Let me know how this works, I guarantee you'll notice a difference.