Heat olive oil in a large pot over medium-high heat and add onion, garlic, and salt. Stir constantly until vegetables have softened and begin to stick, 4–5 minutes. Add red peppers and potato, stirring until fully aromatic and nearly dry, 5–8 minutes. Add fish stock, water, and tomatoes, and bring to a boil. Reduce heat to maintain a steady simmer, cover, and cook about 20 minutes, or until all vegetables are tender. Remove from heat.

Working in batches, purée soup in a blender. Return to heat and add pepper and saffron. If soup is very thick, add a little water to obtain a creamy, light consistency. Bring to a simmer, add fish and shrimp, and stir until pieces turn opaque, 2–3 minutes. Remove from heat and let sit about 1 minute. Add salt to taste. Serve, garnished with chives.