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.