Place onion, garlic and canned tomatoes with juice in a saucepan and simmer 15 minutes; allow to cool a bit so you can puree it.
Puree tomato mixture in a food processor or use a hand-held blending stick in the pan, your choice.
Add pasilla chili powder, diced vegetables, bouillon cubes, fish base and water to pan and stir.
Bring soup to a boil, then reduce heat and simmer, covered, for 20 minutes, stirring occasionally.
Gently add the whole fish fillet, and the other seafood to the pan and continue to cook for another 5-10 minutes until shells on clams have opened and the fish is firm and opaque.
Serve soup garnished with cilantro and serrano peppers, and a wedge of lime to squeeze into the soup.