Please guys, don't repeat the sea cucumber myth. Yes, they are toxic, but the ones you utilize as sand bed cleaners are no more toxic than a dead coral. It is the filter feeding sea cucumbers that are highly toxic and prone to evicerate if threatened.
I personally have an 8 inch detrivore sea cucumber in my tank and my sand is sparkling white. As to the rumor that they are like some toxic little time bombs... I have hermit crabs crawling all over him.... I've had an anemone attach to him... I've even pried him off of a rock before by hand (very dumb move) and he never once evicerated.
Remember, brightly-colored and filter feeding tentacles = bad cucumber
drab-colored with no filter feeding tentacles = detrivore (good) cucumber
Don't give these guys a bad rap. They are awsome lil critters with tremendous cleaning abilities and a very peaceful, nontoxic nature
ps- I have a 3 inch sandbed and the cuke is healthy as a horse.