This shows how to use the Polygon Modeling tools to model a fish. Usually, polygon models are smoothed by placing into a HyperNurbs, but be careful when using HyperNurbs, you should not be placing complex hierarchies of objects into a HN.

The tools used while in Polygon mode here are D (Extrude) and I (Extrude Inner). To make the tails split, I unchecked Preserve Groups in the Extrude Attributes box that comes up right after you select D.

BTW, if you hit Space, you can toggle back and forth between the last selected tools. This is really useful in Polygon modeling to go from the Selection Tool back to the Extrude tool for quickly selecting and extruding Polygons.

If you are following the Step by Step tutorial, here are the steps in order:
Polygon Modeling
Joints
Cloner Object – Introduction
Cloner Object – Using Object Mode to animate
Target Effector
Optimizing Mograph Cloner for Display speed
Random Effector
Timeline
FCurve Offset to Delay Loop start
Specular and Diffuse Maps
Transparency Absorption
Refining Lights and Visible Volumetric settings