Ok I've got a few replies for you (and me as I will try it on my guys too).
So far I have only received replys about salmon & tuna. Some fish can be harmful to them so I will only stick with those 2.

Most feed salmon raw. Although it can be cooked (broiled, poached). Salmon fillets are popular so are the trimmings (sometimes called Nova) Make sure you remove the bones. You can also feed some canned salmon (low sodium or only feed a tiny bit)

So far I have only heard about canned tuna being fed, the low sodium kind.

Because fish is high in protein their stools may be quite runny at first (unless your fuzzie is used to high protein foods).