I have a pencil. Love him! He dose, very rarely move rock, as he wedges inbetween. I also have a pincushion, they both munch corraline, but if you have a stable tank, it will grow back quickly. I would warn, the pincushion will grab stuff and carry it around. This has been the casue of a few of my snails deaths. There are a few pics of my pincushion "giving tours" to various critters and my old hydro, in my gallery. For that reason, I was going to look into long spined ones, I am not 100% sure, but I don't think they carry anything around, like the pincushion. Long spined are venemous, I beleive, so be careful, if you go that route. I love the little things!