How does the burrow even stay intact? I've seen all kinds of substrate used in pistol shrimp setups - pebbles, crushed coral, aragonite sand, fine sand, etc. There's been some people who managed to bait the shrimp into burrowing next to the hardscape, and the rocks sort of "guide" the shrimp to make a very sturdy burrow. Is it better to use fine sand that might cave in, or larger granules?