Why would this concern you more than anything else? If you cook the fish, it's not going to "go bad" as quickly as if it were raw. If you cook anything to a particular temperature and time, it should kill most bacteria in it. Then the acidity of the tomato should help resist bacterial growth a wee bit longer than something pH-neutral. On top of all this, "obvious death" from bacteria in food is some serious hyperbole; it's quite rare to die of food poisoning.