Terrariums and dish gardens require some special consideration. In time, the plants become crowded and some plants begin to dominate. All plants in a terrarium or dish garden ought to be compatible. Cactus should not be mixed with tropical plants. Select plants which remain small or tolerate pinching.
Terrariums have additional problems. They are often overwatered. This is especially true of covered terrariums. It is not true that terrariums never need water, they just need it less often. When a covered terrarium fogs up, take off the cover for a day. The condensation is an indication of too much water in the terrarium. Apply water it in small amounts until it can be seen running into the drainage material at the bottom of the container. The temperature inside the glass will get too high if the terrarium is set in direct sun. Set terrariums in bright light but not direct sunlight.