For conservation workers attempting to restore denuded coral reefs, fish can be both a problem and a solution. They are prone to knocking over young corals newly transplanted onto a coral reef before the cement can harden. But coral researchers are now finding that fish can be allies, too. They can help to clean corals in humanmade ocean nurseries, eating the algae and invertebrates that threaten to smother the young polyps.

Coral is dying across the globe. About 27% of the world’s reefs have already disappeared, according to the World Wildlife Fund, with much of the destruction caused by warming oceans and the coral bleaching they trigger. In past decades, most reef conservation focused on protecting whatever coral was left, but now many teams are taking direct action to repair the damage.

In the Seychelles islands, for example, researchers led by Sarah Frias-Torres raise corals for transplantation, growing branching corals on 20-meter ropes anchored to PVC pipes and larger corals on nets stretched across a square frame. To keep the young corals from being smothered by algae and barnacles, divers must scrub the ropes by hand with toothbrushes, a daunting task for projects involving tens of thousands of corals, says Frias-Torres, an oceanographer at the Smithsonian Marine Station at Fort Pierce, Florida.

She and other researchers had noticed that some fish species are drawn to the coral nurseries, where they eat algae and encrusting organisms. To see how effective these fishy cleaners are, Frias-Torres and her colleagues compared three nurseries that attracted widely differing numbers of fish, presumably because of differences in nearby currents. At the nursery with the fewest fish, researchers had to spend nearly three times as long cleaning, they reported in the African Journal of Marine Sciences.

The researchers also realized that such natural cleaning might help them cope with a problem that arose once they started transplanting their farmed coral onto the reef. At first there were no fish to interfere on the denuded reef. But when the team had transplanted about 10,000 corals, fish started to return.

“They would move their eyes around looking at all the treats that were living in the coral, all those brittle stars and worms,” Frias-Torres says. “They would wait until we had cemented the corals, and as we were looking at what we'd done, they would ram into it.”

By late 2013, the fish were knocking down 16% of the newly transplanted corals. That’s when the researchers remembered the “cleaning stations” that occur naturally on healthy reefs—places where large fish congregate to have their parasites eaten by small fish and shrimp. Inspired, the researchers decided to set up their own cleaning stations where corals could be cleaned by fish before being transplanted.

First, the scientists tested cleaning stations at different depths, and found that fish do the best cleaning job when corals are suspended near the ocean floor. Then, they put their discoveries into practice, taking rope nurseries ready for transplantation and positioning them in the reef amongst the hungry fish. When the researchers returned 2 days later, the corals were clean, and the fish let the divers cement corals to the rocks in peace, they report in PeerJ.

The fish-powered cleaning approach isn’t feasible everywhere, says Ken Nedimyer, founder and president of the Coral Restoration Foundation, which raises and transplants corals in the Caribbean and the Florida Keys. At some sites, there aren’t enough fish to clean corals or to get in the way of transplantation, and Nedimyer’s own nurseries are designed not to need extra cleaning. But at fish-rich sites in developing world, he says, the approach could be valuable.

“It’s a very good idea,” agrees Shai Shafir, a marine biologist at Oranim Academic College in Tivon, Israel, who helped design the types of nurseries the Seychelles team used but was not involved in the new studies. “If you're able to bring the fish to the nursery, they will do the work for you and they will have enough to eat, so it's a win-win situation.”