Eastern mosquitofish
People brought the eastern mosquitofish to Hawaii in the 1910s and to Australia in the 1920s and set it free there.  They thought it would eat mosquito larvae so there would be fewer mosquitoes.  But the eastern mosquitofish did not eat more mosquitoes than the fish that were already in Australia did.  The eastern mosquitofish outcompetes other fish and bites their tails or even eats them.  It can kill larger fish by damaging their fins.  In the western United States, the eastern mosquitofish killed or outcompeted many other fish that were better at controlling mosquitoes than it is.  After people put eastern mosquitofish in a body of water, there may be an algal bloom because the eastern mosquitofish eats or kills all the fish that usually eat the algae.