perhaps not "ideal", but I have a blue-spotted Jawfish in my 20L, when I got him, I made one end of the tank deeper sand/crushed coral/rubble/shells for him, and released him at that end. He chose to setup shop at the other end in about an inch of fine sand. I provided him with a lot of rubble and shells and he built a pretty large pile with a hole in the middle and seems happy.