When it comes to scallops and shrimp and sometimes fish, ask your fish monger if they're treated with STPP. It's a preservative that helps to maintain moisture, not only adding weight but give seafood, especially scallops, a plump looking appearance. Gulf shrimp that are sold fresh typically aren't treated, but a high percentage of shrimp that's shipped frozen is treated with STPP. Oftentimes, shrimp sold at the fish counter is frozen shrimp that's defrosted for the customer's convenience.
My go to food choices for my nems are wild salmon and sushi-grade tuna. I can't prove that STPP is harmful, but I'd rather be safe than sorry and feed foods that I know aren't treated with preservatives. Granted, some tuna is treated with carbon monoxide to prevent browning, but this doesn't appear to have harmful affects.