as i mentioned above-plankton. you can buy frozen plankton, i use this. I also use frozen mysis shrimp (PE mysis) as well as Sallys and Hikari frozen mysis. Also live and frozen brine sjrimp, shavied gulf shrimp chucnks, as I mentioned it pulls pretty much anything from the water column , samples it, then either swallows or spits it out.
AS for sand bed depth, i guess around 4-6". While the books say 1 ft or more, the eels move sideways like eels thru the sandbed. I've been to a few public aquariums that have kept the eels in shallow tanks and they appear to do fine. My eels specifically is in a section of 4", he darts in-an-out w/ no problem