Yes, I had the same dilemma with the muddy substrate when I first started with mine. If you add moss, be sure to tong feed. Never feed on top of it so he won't accidentally eat it when lunging at his food. It causes impaction. I would use live sheet moss if you decide to. I'm not a fan of moss bc of the risk. I have a fake tree plant in mine which works well. If you add some plants they help to hold the moisture in like spraying under an umbrella. The moisture drops linger on the underside of them.
I had to fiddle a bit when I first started with my pac tanks. I actually ended up having glass cut to 3/4 the top of my tanks. The glass cutters sanded the edges. It works very well. I just can't put my lamps over it or they glass will crack. I measured the area so I have a little over an inch around the lamp for the glass to clear the hot areas.
Tip for you...to keep your substrate from growing mold and anaerobic bacteria, every few days turn it. Flip the bottom substrate to the top and stir it around the tank. This puts oxygen in the bottom, which the anaerobic bacteria do not like, decreasing the speed at which it multiplies. It also distributes the moisture evenly.
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