Place the fish bones in the water and bring to the boil. Simmer for 20 minutes, then pass through a strainer lined with muslin.
Return the stock to the pan and add the vegetables. Bring to the boil and simmer again for 45 minutes. Strain again, then use or store. You can store stock in a covered container in the fridge for up to four days.
You can keep it indefinitely if you re-
1.5kg/3lb fish bones, including heads
1 large carrot, chopped
2 celery stick, including the leafy top, sliced