Fry onion, peppers and garlic in olive oil in a big saucepan for a few minutes; add hake and mussel meat and fry a bit longer.
Add potato and stir through, then top with boiling water / stock until just covered. Season and add paprika.
Bring to the boil, then turn down the heat and simmer, partially covered, until fish and potatoes are almost done.
Stir occasionally, but take care not to break up the fish too much. Towards the end, add chopped parsley as well as pasta shells, if desired. Check seasoning.
Cook till pasta shells are done and serve.