Not sure exactly how much this will relate to amphibians but I recently got in a fairly heated discussion on a garter snake forum concerning feeding fish to garter snakes. It's a common practice in the garter snake world because fish makes up a fairly large percentage of garter snakes' diets, but I think you need to be weary of any fish that comes packaged. (Pre-frozen) Fresh or truly fresh fish should be okay, but many packaged fish products are treated with a sodium compound preservative called Sodium tripolyphosphate, which is harmless to humans but is used to keep frozen fish looking appetizing after it's been frozen.

This compound is actually a neurotoxin and is commonly found in soaps and detergents. It can kill garter snakes and causes them to have seizures, convulsions, and eventually die. These symptoms in garter snakes are similar to those of dehydration which may be explained by the sodium being taken in with the fish.

I'm not saying that this compound will certainly harm amphibians as well but knowing that frogs are generally more sensitive to harmful compounds than reptiles, I would either avoid feeding fish or be very confident the source of your fish does not use any chemicals on the products