I helped students build several terrariums in the 10th grade biology setting. We found a little mold went a long way. You could organize the set up of several terrariums and see what results in a desirable outcome. Some might argue desirable is that the plants grow well. Some might define success differently. All is worth discussing before construction. I think we had the best discussion after watching mold take over one of our terrariums. It was not a pretty tank.