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Every 50th kill of this enemy drops Corrupt Goldfish Banner.where they come from
Three routes lead to the same fish. Corrupt water produces them on its own, which its land equivalent cannot do, so a corruption world carries a standing population without anything happening. A blood moon flips any goldfish it finds. And Vile Powder does the same job whenever you feel like it, which is the route you control.
building a farm
A Fish Statue running during a blood moon spits them out already corrupted and saves you the powder entirely. Two limits to know first. None of them drop Bloody Tears whichever way they spawned, and statue spawns carry no coins, so what you are building is a drops and banner farm rather than a money one.