Lionfish are invasive species in the Atlantic ocean that can harm coral reefs and certain species of fish, for example the algae-eating parrotfish, which prevents seaweeds from overgrowing completely.
Lionfish were introduced into the Atlantic due to aquarium owners not wanting the fish in their tanks anymore due to their carnivorous appetites, and thus they started collecting all over the Atlantic from the southeastern United States coast to Florida and North Carolina.
As stated in the first paragraph, Lionfish pose serious threats to coral reefs. An example is how the Lionfish eat the herbivores. The herbivores eat the algae and seaweed (like the parrotfish), keeping the plants down and controlling populations. However, if the Lionfish cause the parrotfish to go extinct, there could be a major disruption of the food web.
Some researchers, divers, or just regular citizens are trying to keep the lionfish problem in check by spearfishing them, or even catching them by hand or in nets. People go out on boating expeditions for just catching these problematic lionfish.