In a medium bowl, crush the tomatoes and their liquid with your hands. Strain the liquid into a small bowl.
Meanwhile, in a large saucepan or medium dutch oven, heat the EVOO over medium-high heat. Add the fennel and scallion whites and cook, stirring occasionally, until lightly browned, 8 minutes. Add the garlic; cook for 1 minute.
Add the strained tomatoes and cook, stirring occasionally, until the mixture is thick, about 5 minutes. Add the potatoes, strained tomato juice and 4 cups water; season with salt and pepper. (Add more water if necessary to cover the potatoes.) Simmer until the potatoes are tender, about 15 minutes.
Gently stir in the fish and scallion greens and simmer until the fish is cooked through, about 5 minutes. Garnish with the fennel fronds.