Re: 20 GAL Mantella Tank
Your frogs look really gorgeous!
One quick tip about your orchid (the Phalaenopsis)- most Phals are not terrestrial, so you will have better luck with it lifted up off the ground. Otherwise, it's likely to rot, since that foam cup it's in is probably going to break down faster than you expect. Keeping it in a plastic pot buried in the substrate won't give any real drainage or airflow around the roots, and it will be hard for you to keep an eye on how it's doing.
Phals are also susceptible to crown rot, which happens when water sits too long in the base of the leaves, especially overnight. You might want to mount it on that piece of cork log in there, and be careful when misting, so that you only spray the roots. It's a little tough with a short tank, since the inflorescence can be quite tall.
You did a nice job on the background, and did well with not stuffing too many plants in there. It's going to look even better as it grows.