I am making ottolenghis Tunisian Fish and Fennel soup.Will I ruin it by making a fish broth from salmon heads? Have read not to use Salmon when making

Salmon heads can make a nice, rich broth, it'd be pretty distinctive, not really my vision of a Tunisian flavor profile. I'd use something milder. I see that Ottenghi himself suggests chicken broth as an alternative.

But, while I might use something else for that recipe, you might want to give it a go. I can report using salmon and Atlantic bluefish (even stronger!) in places that they supposedly wouldn't work. The results are never traditional but often tasty.