Just beware that they black spiny urchin will grow faster than you think. I had one, and got him because he was cool, not to take care of algae, so he just ate whatever, never supplemented him. I brought mine back to my LFS
because his spikes were 5" long themselves, and it was hard to get my hands in the tank in any area it was in. I was stuck once, and that was enough those spikes hurt. I couldn't believe how quickly it pointed those spikes toward my hand.... By the way, def. not an anti urchin post, just a warning about the black spiny in a smaller tank. I could see myself getting one in a 125G +. I have a tiny tuxedo in tank (which is growing a lot slower than the black did) and a bigger collector urchin in my other tank. If I find a small pencil urchin, i plan on getting that for my 20G since it is currently urchinless.