Hi PhxJosh,
Slimer gave good advice. If you intend to use any substrate, bear in mind they do make maintaining the tank slightly more difficult (need to siphon off detritus among substrate).
You have two main options if you want to use substrate. Sand will be best substrate to use. This includes pool filter sand, freshwater aquarium sand or children play pit sand. Particle size of the sand grain should be 2mm or less in diameter.
The other option is the use of large pieces of slate. In this case, make sure the type of rock you are using are firstly safe to use in freshwater tanks (doesn't leach out, alter pH etc) and that the pieces are way to big to be swallowed by the axie. Due to the difference in axie sizes, it would not be safe to rely on a specific numerical value for the diameter but use your judgement. You want the rock pieces preferably about 3 times the size of the axie's head to be safe.
Cheers.