Sea urchins are considered by marine ecologists to be keystone species in tropical seas, as they promote the development of reef-building corals via the intense grazing pressure they exert on benthic algae. As such, they are an indispensable herbivore in any reef aquarium clean-up crew. Highly valued for its beautiful appearance as well as for its ability to eradicate filamentous algae, the urchin has long been a reef aquarium hobby favorite. Though less active during daylight hours, it emerges at night to feed on a wide variety of algae. Particularly in smaller tanks or where benthic algae growth is minimal, it will eventually require supplemental feedings in the form of dried algae or offerings of live material from the refugium.
AlgaeBarn sells two species of urchin. Both of these urchins are great algae eaters and look awesome, but one might be better for you than the other.
The Tuxedo Urchin is a fantastic invertebrate to have on your clean up crew. A group of them will decimate the algae in your aquarium. The advantage to the Tuxedo Urchin over the Pincushion Urchin is its small size. This urchin is far less likely to knock over frags or rock work. This urchin will generally be better for smaller tanks or tanks will lots of small coral frags.
The Pincushion Urchin is a larger urchin that is even better at eating algae than the Pincushion Urchin. Although this urchin can be more destructive than the other, it eats algae much faster. It also has an appetite for hair algae, which Tuxedo Urchins do not. The Pincushion Urchin is best for fish only aquariums (as long as there are no fish that will eat it), larger aquariums with lots of room and sturdy rocks, and aquariums with well-established coral colonies. If you have a hair algae problem, the Pincushion Urchin is one of the best ways to get rid of that issue.
Both of these Urchin species are completely reef safe. They will not eat corals. Again, the Pincushion Urchin can cause a coral frag to fall because of its large size, but it will not intentionally harm corals.
As for which of these urchins is more beautiful, that is up to you to decide. Most hobbyists generally agree that Tuxedo Urchins are more desirable when it comes to looks, but there is some appeal to the spiky style of the Pincushion as well. Tuxedo Urchins are cool because they come in two different colors: blue and red. Being able to pick a color adds a fun element to the Tuxedo Urchin.
Either way, both are fantastic urchins and fantastic invertebrates to have cleaning around your aquarium.