Soak clams in 1 litre of cold water mixed with 1 tbsp salt for 10 minutes to force them to release any grit.
Season salmon with ¼ tsp salt. Season shrimp with ¼ tsp of salt if using raw shrimp (the cooked kind has salt added).
Heat oil in a large pot over medium-high. Add onion and a ¼ tsp of salt; cook until translucent and soft, 8-10 minutes, stirring occasionally.
Add garlic and red pepper flakes; cook 2 minutes.
Add tomato and cook 2 minutes to until soften.
Stir in broth, clam juice, wine and 1 tsp more salt. Cover and bring to a simmer.
Add clams, fish and shrimp and cover pot again. Cook until clams open and fish is just cooked through, about 5 minutes. Stir in butter and parsley and serve right away.