Place the fish head and bones in a large stockpot. Add the cold water, bring to a simmer, and cook for 5 minutes, skimming any foam from the surface.
Add the carrots, celery, onion, and tomato.
Continue to simmer for 20 minutes.
Strain the stock, season lightly with salt, and allow to cool to room temperature.
Refrigerate or freeze until ready to use.