If you meen on a trip from Japan, that would be quite amazing if you could do it. First, you need an import/export license from Fish and Game, then you need to get a permit from the USDA, then you have to have someone in Japan arrange for an authorized Vet inspection and the necessary paper work, then you have to make sure it gets on your flight and finally when you get back to the US you have to come into an authorized port of entry and have an Aphis vet inspect you paper work and your import broker needs to clear the fish through USDA, Fish and Game and then Customs.
Good Luck.