Got a fish head, bone, taill? Do not waste them, use them or give them to me.
Use a pair of wire cutters to remove the teeth, fins, etc. is easier than anything else. Remove the gills.
Sprinkle a little salt over the skin and mucous layer.
Use a spare knife to remove slimy stuff inside and outside as much as you can.
Wash them with hot tap water and clean again. You can use a new rough nylon or polyester kitchen sponge to clean the surface.
Put them all in a large pot, with one or two yellow onions, a few garlic cloves, and a little salt. Cover with boiling water. Bring back to a gentle boil then simmer for at least 20-30 minutes taste and adjust seasoning with more salt, fish sauce, ground black pepper and a little bit of sugar. Voila we have a fabulous fish stock. Use the ratio of water, salt, fish sauce in the link of tomato soup with fish stock.
Add green like spinach, zucchini, curly mustard, etc. to make veggie soup "Canh Cai" or make sweet and sour soup with tomato, pinapple, tamarin, okra, beansprout, etc.
Recipe for tomato soup with fish stock is here