I have three cucumbers in my tank. They have a very primitive digestive system. They pull the ditritus filled sand in one end, and it moves through to the other end and comes out nice and clean. From the reading I've done on them, they do little to no harm to the fauna you want to keep in the sand. They are doing a pretty good job. I would not recommend a sand sifting star. They eat everything, including the pods and other beneficials you want to maintain. And once they've depleted their food sources, which they do rather quickly, they starve to death. Nassarius will come out and feed on the ditritus on the surface before it has a chance to decompose and get into the sandbed. Blueleg hermits are also good for that.