In a medium Dutch oven, heat the oil over low heat. Add the onion and bell pepper, season with salt and black pepper and cook, stirring, until softened, about 10 minutes. Stir in the tomato paste and cook for 1 minute. Stir in the cayenne and cook for 30 seconds. Stir in the crushed tomatoes and 1 cup water, season with salt and black pepper, increase the heat to medium and cook until heated through, about 5 minutes.
Add the tilapia fillets one at a time and simmer until the fish is flaky and cooked through, about 10 minutes.