I was trying to fold a two coloured cube, and the result was this fish.
Start with two triangular pieces of paper (different colours if you want) and put them together to form a square. Don't glue them together, they should just lay loosely next to each other.
So what you have to do now is to fold a cube starting with the Waterbomb base, with this square sheet of paper. Be careful, because at the beginning the two pieces go apart rather easily. You should have folded the cube sometimes before you try this model.
It is easier to open the fish than the cube at the end, because you have one open end (the mouth of the fish).
Now you just have to cut out a fin with a long thin end to put it into the hole at the end of the fish. If you make the thin end longer you can use it as a tongue. Draw eyes on the fish to complete it.
The finished fish. You can open and close the mouth by pressing and releasing the sides of the fish.