Add the tomatoes & couscous. Stir in 2 cups of the shrimp broth. Bring to a simmer and then reduce to medium and cook, stirring, until most of the liquid is absorbed, 8 to 10 minutes. Stir in another cup or so of the broth, return to a simmer and cook, stirring, until most of the liquid is again absorbed, about 8 to 10 minutes. Stir in the remaining cup of broth and cook, stirring constantly, until the couscous is tender.