If you're looking into a spider decorator crab, i would have to say look into something else. speaking from personal experience it is not worth the hassle. don't get me wrong it was kind of cool at first having him eat from my hand and watching him decorate himself with everything fom sand, shells,algae, sponges, and polyps. but the novelty soon wears off when it completly decimates any zoanthid colonies you may have along with any other polyps it can get ahold of. i lost all of my colonies of zoas to it. not to mention unless you watch your tank at night with the moonlights, you will never see it. most of the time i had to wait an hour or so after all lights went out just to feed it as it would not come out during feeding time. also if you have any kind of water flow in your tank you have to hand feed it because it moves so slow it cannot catch the food.
all in all i was actually releived when i found it belly up in my tank a couple of months ago.