When my daughter told me that she wanted a fish cake, I thought, how in the world am I going to do that? After calling a custom cake bakery and discovering that it would be $50+ to have one made, I was determined to do this myself! Fortunately, your website came to the rescue!
I modeled this cake after Cake "Fish 20". I happened to already have a small oval cake pan (a Wilton pan from Michael's), so I started with that. I made this a two-layer cake so I made two of the oval ones. Then I made a 9x9 cake and basically just cut it apart to get the fins and tail.
Once I cut a shape (like the tail) out, I put it back on top of the cake to cut another one identical to it for the second layer. (I just used frosting to stick the two layers together.) I cut the tail first since that was the largest and then did the fins with the leftover smaller pieces.
We used toothpicks and frosting to get it all to stick together. I decorated the eye (an M&M "glued" with frosting to a circle candy), the smile (a cut of the striped candy that you see elsewhere on the cake), put in the line of M&Ms around the face and then told my 3 year old daughter and 4 year old son they could go to town decorating anything to the right of the line of M&Ms. That worked great because they could get involved without me interfering, but I didn't have to worry about the recognizability of the cake.
I added the blue lines to the fin and tail with frosting. (You can see the squeeze tube that I used just to the right of the plate.)