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You can add it directly to a tank with fish in it, but if you're still worried about harming your fish, about half an hour wait would be more than enough time to allow the dechlorinator to do it's job.
Welcome to AA! Is it a new tank? Are you familiar with cycling? It's the single most important thing you'll ever do with your tank. Check out the link in my signature and feel free to ask any questions you have.