Sea urchins can make great algae grazers, but they can also have several drawbacks that most potential urchin owners should be aware of. Pincushion urchins, for example, stay small and crawl all over the tank in search of food. Unfortunately though, they take any loose frags with them. Longspine urchins are very attractive and are also effective herbivore grazers, but they get massive and their spines are very sharp. Just lightly brushing into this urchin while doing aquarium cleaning can leave you with fingers full of sharp needles. Other urchins can knock over large pieces of live rock, crushing corals or severely scratching your glass.