There are a lot of different eels, most like smooth small gravel, some like mud or sank, american eels will do fine with standard gravel, esp if you give them nooks and crannies to exlpore, they can also be kept in small tanks for their legth, I kept a 36" one in a long 55g with no problems other than needing to put a latch on the tank and siliconing the filter to the tank, all eels are escape artists. If you like true eels and want a smaller one go with spaghetti eels, big get the marmota, and middle sized american eels.
