As someone who consumes a lot of nori alongside my fish lol, I'd say toss it. It's cheap enough to get a new pack and it does get stale even if kept dry.
What brand do you feed your fish? I get “far edge aquatics” off Amazon. $23 for 5.1 oz…which is a lot. Also, what brand do you eat? I get overwhelmed a Jong Boo there are so many choices. I could spend an entire day in that place but my wife and kids are always waiting in the car when I run in to get the sashimi platter.