Briny clam juice is the secret weapon in this dish. We like the salinity of Bar Harbor brand, which goes well with the paprika.
Heat oil in a large straight-sided skillet over medium-high. Add fennel, celery, and a pinch of salt; cook, stirring often, until vegetables begin to soften, 5 minutes. Add garlic and paprika; cook until fragrant, 30 seconds. Add tomatoes and cook, stirring, until thickened slightly, 5 minutes. Add clam juice and potatoes; season with salt and pepper. Bring to a boil, then reduce heat and simmer, covered, until potatoes are fork-tender, 30 minutes.
Season fish with salt; gently fold into stew. Return to a simmer, cover, and remove from heat; let stand 5 minutes. Serve, topped with fennel fronds, a drizzle of oil, and more pepper, if desired.