I can only tell you about a Pencil urchin we had. Ours was pretty cool with the exception of starting to munch on our coralline. I thought maybe the coralline growth would keep up with it, but I was wrong. We dont have the most elaborate set-up so the coralline we finally got growing was kind of precious to us so we moved it to the 10g. Unfortunately, it continued eating away. Didnt know what to do with it til we found out the LFS we got it from would take back and give a discount on new livestock. Ended up getting hairy and green shrooms at a decent price from the initial $7 investment, not a bad deal; probably at least a $10 savings on the corals. Anyway, they're cool as far as eye-candy, but ours turned out to be not so reef-safe. I'm sure there are other experiences out there, so dont judge all urchins by our exp. Maybe we didnt have enough green algae for it, I just dont know. Personally, I wouldnt recommend them, but thats just from our 1 exp. Oh yeah, ours hid a lot for like a month (nocturnal maybe), but eventually started wandering around the tank in the open; never did see it on the glass:thinking: