Wood Frog does seem likely- the black behind the eyes and under the armpits plus the small size at metamorphosis. Do you know where the shipment was from or where the goldfish originated? Pictures of it as a tadpole may help ID as well.

Whatever it is, it should have a land area that is easy to get to now that it is a froglet (this is also critical for the remaining tadpoles once they get front limbs- drowning is very possible at the 4 limbs+tail stage). Fruit flies, spring tails and pinhead crickets should work as feeders.