Urchins

Urchins are the heavy machinery of a clean-up crew. Where a snail grazes the easy film, an urchin scrapes rock bare, and that is both the reason to buy one and the reason to think twice.
What they actually eat. Tuxedo and pincushion urchins work through hair algae, film algae and leftover food, and they get into crevices a snail cannot reach. Slate pencil urchins do the same but will also graze coralline, so if you have spent two years growing purple rock, a slate pencil is not the animal for you. None of them are a cure for a nutrient problem — an urchin eats the symptom while the phosphate that caused it stays exactly where it is.
What they knock over. An urchin is strong, blind to your aquascaping intentions, and busiest at night. Frag plugs that are not glued down end up on the sand. Loose rock gets shifted. Tuxedo urchins in particular decorate themselves with anything portable — shells, algae, and your unattached frags. Glue everything before you add one, and expect to go looking for the occasional coral in the morning.
What they need. Stable salinity above all else. Urchins are echinoderms and they do not tolerate a swing, so acclimate slowly on a full drip and never a quick dunk. Once settled they are very hardy and live for years, but they want a mature tank with enough algae to actually sustain them; a spotless new system will starve one.
If you want a tidy tank rather than heavy algae removal, snails and hermits are the gentler option — see our clean-up crew guide. Acclimation detail is in the acclimation and dipping guide.