I thought it was a fish too on first glance, but if you look a little more closely what appears to be the 'body' of the fish is just the cast shadow from the tail. The dark streak could be feces, or it could be an external gill. Many tadpoles have external gills when they first hatch, and they often resorb them more quickly on one side than on the other.
Just my two cents.
Just my two cents.