the sand bed isnt that important for the eel, 3 or 4 inches should be fine, i dont know of anyone that has a deep sand bed for eels, having tunnels and caves is the most important feature.
What i mean by messy is, yes, they add a very heavy bio-load when they get larger, and having poor eyesight could cause problems when working in the tank, you already have to watch out for the lions and possibly a foxface so? its personal preferance.