The topic of filtration, sandbed and other tank needs, as well as Mantis types and aggressiveness have been covered many times... searching or digging around will probably yield more and better info than I'm willing to type out while I'm brushing my teeth.
I have a 32G tall with Peacock Mantis in it. There's about a 4" sandbed in the entire thing, and he dug down to the acrylic-lined bottom inside his burrow. For filtration, I have about 60lbs of LR in there, and I use a Bakpak 2r for filtration/media/skimming.
On the way I have a 3G pico in which I'll have a juvenile Viridis in for some time.
Do lots of research on lighting, burrows, sandbeds, food, molting and over-all mantis care before you commit to anything. I researched for almost an entire year and let my tank cycle for a ridiculous amount of time while I figured out exactly how to care best for my new buddy.
I had to save a mantis from a friend temporarily because he hastily setup a tank without researching much. I nearly kept the little dude, but the guy showed he was able to learn from his mistake after nearly killing the mantis.
Take your time. Your Mantis will thank you and will be more active and comfortable in a better environment -- I know it sure has paid off for me.
