Sea urchins
Voracious herbivores that control algae in the reef. Useful but can damage corals with thorns and move fragments.
Detailed explanation
The Diadema setosum (black urchin with long spines) is the most effective algae eater on the reef. Cleans rocks impressively.
The problem is that they are bulldozers: they move coral fragments, they can damage corals with their thorns and they are difficult to remove once they have grown.
Pencil hedgehogs (Eucidaris tribuloides) are smaller and less destructive. Blue hedgehogs (Echinometra) are a good middle ground.