Preheat the oven to 375° F.
Place chicken stock into a pot over medium-high heat. Bring to a boil; add saffron and salt. Turn off heat, and allow to steep.
In a large paella pan or skillet over medium heat, add oil. Then add onions and bell peppers. Cook for 7 minutes. Add garlic and sausage, and continue to cook 3 more minutes. Add rice and cook, stirring often, for 4 minutes or until lightly toasted. Add stock mixture and tomatoes. Cover with lid, and simmer rice for 15 minutes.
When rice has absorbed most of the stock, arrange catfish, shrimp and mussels. Replace lid, and cook for another 6 to 8 minutes, or until rice is tender and catfish is cooked.
Notes: You want the bottom of the rice to be slightly caramelized. Do not eat any mussels that do not open.