I highly recommend the use of fine-grade sand for mastacembelid eels. It seems to prevent or ameliorate any damage to their sensitive, mucous-laden skin. While they may spend much of their time with just a head sticking out of the sand, this ability to hide whenever and wherever they want cuts down on conspecific aggression and the stress of captivity.
Drop in a clean trout worm if you want action anyway! heehee