For goldfish, it's go big or go home.
Researchers have found giant goldfish living in three saltwater estuaries in southwest Australia.
See also: Big fish steals rod from man
Some have grown to more than a foot in length and weigh more than four pounds.
The invasive species has harmed the ecosystem by eating vegetation and the eggs of other fish.
They could also introduce new diseases into the water.
Scientists say they escape when people flush them down the toilet and adapt to the open water.
Researchers worry that the growing fish could spread to other rivers and wreak further havoc on Australia's waterways.