The need for a substrate of a minimum depth is probably the most readily available information you can find on keeping garden eels in an aquarium. This is indeed a very important topic and I want to go into some detail on what I've found to be most effective. There seems to be a consensus that an 8 inch deep substrate is the minimum that should be provided as a home for your garden eels. This depth is mostly derived from these 2 facts: the Spotted Garden Eel can reach a length of 16 inches and they generally keep ½ of their body inside the burrow (Shedd Aquarium 2011). Generally, Spotted Garden Eels are offered in lengths well short of their 16 inch maximum length and their captive length is likely to not exceed 11 inches since the maximum length usually given is reflective of the largest specimen encountered, not the average length and work by Jay Hemdal has estimated that 2/3 of the maximum size is a more accurate predictor of maximum captive size (Hemdal 2009b). Since the Spotted Garden Eel can and does retract completely inside their burrow either when seeking protection or during the tank's dark period you might think that a 12 inch substrate depth might be a more ideal depth and if you have a tank tall enough to accommodate a 12 inch deep substrate then I would recommend that as an ideal depth. I will note that absent of a substrate depth equal to the eels length, the eel will still retract completely inside their burrow by extending their burrow parallel to the tank's glass bottom.