What kind of fish do you have? I've always used garlic rather than garlic juice...ie crush some garlic, put it in tank water with the nori for a bit then add it. The non-roasted will contain more nutrients for your fish so i'd suggest giving that to them over the roasted variety. But if you don't have omnivores to begin with they won't go for the nori. And even if you do, they still may not, my purple tang loves spirulina, but doesn't care at all about nori--go figure.
-leigh