Urchins make for great members of the aquarium clean up crew, especially those of the pincushion variety. They stay small, their spines are not sharp, and they often show up with some funky coloration (e.g. the blue tuxedo urchin). Despite the fact that urchins are constant grazers of various types of algae, they can cause some issues in the aquarium. Pincushion urchins are pretty good at camouflaging themselves by using things from their environment to cover their bodies. While this is a totally cool behavior, aquarists quickly realize that this means coral frags will often be used as personal urchin decor. In addition to frag relocation, these urchins can also knock over small rocks, causing other issues in the process.