Snowflake Eel is a cool fish, he stays in his den for the most part, they are nocturnal predators so they "sneak up" on the fish at night, usually just cruises by them, sometimes if the fish gets near his opening to the den he kinda comes out, but I dont think he will do harm - If he wanted to eat the fish, he would eat it.
So I keep him well fed, he is hand fed mysis untill he retires back into his den the fish pick up the rest flying debris and late at night he cruises along the sand bed picking at left over food then goes back into his den where he keeps his head out.
They can get up to 36 inches in a big tank, 24 in a smaller they grow bigger with a bigger tank, I more then likely wont keep him forever, probably trade him for some cool corals.
If your using uncured rock, it might take your tank up to a month to cycle.