The fish tail looks similar to a regular braid, but is more intricate. You can do this at the end of a ponytail with all the hair or pigtails. I started with a front piggy with the hairband at the part (rather than in the middle of the section) and then did a pull-through with a topsy tailer.
Divide the tail into two pieces.
Beginning on the right (it doesn't matter which side you start on, but the pictures start on the right) pull out a thin piece from the side.
Cross the thin strand over top, and add it into the left section.
Now take a thin strand out from the left side at the outer edge.
Cross that over and add it into the right section. Pull another thin strand from the right side edge.
Cross it over and add it into the hair on the left.
Repeat, repeat, repeat! When you have done this several times, you will come down the braid to where you find sections of hair that have already been use higher up in the braid. You will see places that seem to naturally divide because they have already been divided earlier in the braid. If you use these same pieces, it will make the braid look quite neat on the underside, but it really isn't necessary to worry about it. You may disregard these pieces and just continue to pull off any section from the edge.
These pictures are here to show how the hair sort of naturally breaks into sections after you get down a ways.
Keep following this pattern until you run out of hair to braid, then fasten it off.