Or fizzy pop bottle fish if you're from where I'm from. We made about 12 of these over the month leading up to my older daughter's pirate-superhero-mermaid party. I had to cut the fish out of the bottles with a pair of large scissors and an exacto knife, but the kids all got to paint and decorate them together to make a mermaid tropical lagoon under the play structure bridge at the park.Simply cut the neck of the bottle out to make the fish mouth. I then glued one of those little paper condiment cups into the hole to make cute little fish lips. Cut the bottom off the bottle and then squash the sides flat onto each other to cut out the tail and fins. Use a couple of bits of tape to hold the tail together.
Here's a photo showing how I cut up the big 2 litre soda bottles (I raided friends recycling bins again for this project, so thank you to anyone that helped source all the pop bottles for me). I tried making a couple of fish out of smaller bottles, but they just didn't work quite as well and it was nice for the preschoolers to have a big bottle fish as a canvas, rather than something smaller and more fiddly anyway.
I cut out a load of construction paper into scale shapes and the kids glued them on, some very tidily and some willy nilly, but when they were all done they made a very cute reef full.
Here's the lagoon under the play structure with the spent piniata.
My kids loved the fact that they looked like the fish out of the book "Rainbow Fish", which is one of thier favourites.