I have always had a yellow sand sifting sea cucumber in my tanks. Of course it's easy for me, I can collect them when I go snorkeling in the Florida Keys. And I had one die in a small 35g tank and it didn't cause any issues with the rest of the tank. The one in my 40g cube is about as big around as my thumb and varies from 4" to 6" long depending on what he's doing when I see him. And he will disappear into the sand and I sometimes won't see him for a week or more. I have 2+" of sand and almost no rocks in the sand due to an egg crate platform just above the sand that my rockscape sits on. It's an unusual tank set up. You can see it in my tank build thread and there is a link to it in my signature below.