If they are garden eels, they will almost never try to escape if the sand bed is deep enough. When they don't have a deep enough sand bed they are uncomfortable and try to find a better place to live, in your tank that means up and out. They require at least 10 inches and 12 would be better. They can live in as little as 6 inches, but they wont be happy at first and may try to move. The other problem with shallow sand beds is they try to burrow through the bottom of the tank and can damage the end of the tail which is what they use to dig. If this happens they almost always get infections and die.