Cut off the tail and head then put them in a sauce pan.
Dice up the onions.
Add the onions, paprika, optional salt and black pepper to the sauce pan.
Fill the sauce pan up with water until the ingredients are submerged and bring it to boil.
Simmer it for 60 - 90 minutes until the meat easily separates from the bones.
Separate the hard parts from the liquid then start the meat extraction process.
Mash through the meat of the fish on the holes of a sturdy sieve. It's a heartbreaking toil so better get a special equipment designed for pureeing things. Even better if it's electric. An immersion blender may come handy but that also means we'll puree the fish bones too.
Pour the pureed fish back into the liquid and we have a delicious fish stock ready.
To make it more appealing put the fish stock back on the cook top, add a some more paprika and some sliced up fish pieces.
Bring it boil then cook it on medium heat until the fish is done, about 5 - 15 minutes depending on the width and type of fish we use.
Notes
As fish stock being a labor intensive and quite messy task it's recommended to do bigger batches and freeze the stock for later easier use.