Bring a large pot of salted water to a boil and cook the pasta according to the package instructions. Once cooked, drain the pasta and save 1/2 cup of the cooking water.
In a large skillet, heat the olive oil over medium heat and add the diced bell pepper and minced garlic. Cook for 3-4 minutes until fragrant and soft. Add the tomato sauce and let simmer for 10 minutes.
In a small bowl, combine the ricotta, Parmesan, and half of the mozzarella cheese and mix well.
Add the cooked ziti to the skillet with the tomato sauce and reserved pasta water. Mix together until well combined.