You can use any gravel at the bottom of the pond, they don't try to eat things on the bottom of the water. One way I set up my tank once was to put about 5" of water in a tank, a filter, and made the land area out of rocks, a few big rocks as a foundation (hide the filter in there), and a big flat rock over the top. Stick a couple pothos or bamboo (or both) in the water, wedged in by the rocks, and you have a fire belly tank. I did this because the dirt/moss kept dirtying the water, the water/rock setup eliminates that. Crickets die easily in this setup, but I was using mostly lobster roaches at the time (roaches are smarter than crickets).

Right now, I have a different setup. I have about a 5" deep bowl (about 10" across) to make a pond, and land area made out of big gravel, and a couple fake plants stuck in the gravel, and some moss. Every few days, scoop out some water, and add fresh. They like this setup, the seem to like more "land" and it's more cricket friendly.