http://i.imgur.com/J41DP7J.jpg
Mostly I would do this to maximize space but also ease of cleaning. You can super glue anubias nana to the rocks and use that instead of substrate - makes a nice carpeting effect, holds down debris aka poop but also easily removed for cleaning. Anubias barteri, a larger leaf anubias is on top of the rock. You can glue, tie or simply wedge it in place. Again, easily removed for cleaning. As mentioned before anubias doesn't require high light or fertilizers. This also gives your puffer more space in 2 ways - no substrate to displace water and also utilizes the center of the bowl while still giving you a centerpiece and creates an interesting structure for your dp to explore. Because there is no substrate you can easily re-arrange this setup - reconfigure the arch, move the plants ect - any time you feel your dp is bored and you want to give him something "new" to explore.
For fun here is a pic of my red-eye, Shmendrick, showing off how much he likes anubias lol. Taken about 5 minutes ago
http://i.imgur.com/gNLRJN0.jpg