Here's what I came up with. I googled "fish costume" for inspiration... ok well I first googled "fish costume" to see if I could find a cute affordable and good quality costume for sale, because I really haven't ever made a costume before, but no such luck. So with a few photos in mind I went to Walmart and bought a couple of yards of felt and some ping pong balls.
I spent at least an hour cutting out scales by hand, but my husband and I are on a Battlestar Galactica Netflix kick, so I cut them while watching those and the time went by quickly.
I used one of my son's sleeveless summer onesies as an idea for the general shape, but I made the costume much larger than the onesie to fit over any cold weather clothing he might need to wear. Then I cut the body out of felt and sewed on the scales. I used some of my daughter's glittery tulle left over from her tutu to give the tail some shimmer. The eyes are ping pong balls cut in half with sharpie marker circles on them. I used hot glue to attach them to the cap.
The basic shape is this- A tank top like shirt with a tail in the back. I just winged it on the fin size and shape in the back and on the head piece.
here to see another blogger's version- inspired by mine. Very cute.