Kanziding Fish Market is northern Taiwan's largest fish market, with roots going back to the Qing dynasty. It is most interesting very late at night, when the wholesale trade is active and fish auctions fill the market with movement.
Come around 2AM to 4:30AM if you want to see the auction side rather than just the surrounding seafood streets. It pairs naturally with Keelung's harbour food scene, but it is a working market, so expect wet floors, noise, and fast-moving vendors.