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Every 50th kill of this enemy drops Vicious Goldfish Banner.where they come from
A Blood Moon over a Crimson world flips the Goldfish that spawn along the shore, and in current versions Crimson water produces them with no moon involved at all. Vicious Powder does the same job to any goldfish on demand, which is the option that works in daylight.
farming them
A Fish Statue on a wire puts out these instead of ordinary goldfish for as long as a Blood Moon is up, which is the controlled way to fill in the bestiary entry or grind the banner without walking a dark coastline hoping for spawns. Do not count on the night paying for itself through them, though. They cannot drop Bloody Tears, so nothing they give you feeds back into extending the event.