I wouldn't be as worried about the sodium as I would the potential metals..
Also, shrimp is way different than fish (I ran a DNA gel of shrimp vs several types of fish awhile back; it was actually really interesting!) but the protein content and all is way different. It might not be as nutritional for them...
There's so many cheap food options that we know are safe, it'd just be easier to avoid the shrimp IMO...