Step 1:
This is what you need to make the aquarium:
4 blue papers of about 11x11 centimeters
4 blue papers of about 8x8 centimeters
4 blue papers of about 6x6 centimeters
8 light blue papers of about 3x3 centimeters
glue and/or tape
a piece of ribbon or rope 

Step 6:
Use the width of the stick as guidance to prefold the smaller papers (which are also rolled up into a square stick)

Step 5:
This is how to model should look.

Step 2:
This is a paper of 11x11 centimeters.
Make Zig Zag folds over these lines.

Step 4:
Make a square shaped stick of the model, use glue or tape to close it.

Step 3:
Fold these lines into the right direction

Step 10:
The four shortest sticks are glued between the bottom and top part

Step 8:
This picture shows how the bottom and top part of the aquarium are made.

Step 9:
The bottom and top part are exactly the same.

Step 7:
Make 4 sticks of each size, this makes a total of 12 sticks for one aquarium.
All sticks have the same width.

Step 16:
Fold the corner inwards, see yellow lines.
Do the same on the other three corners, see pink arrows.

Step 15:
Use the width of a stick as guidance for these folds (prefolding).

Step 12:
Add glue to the top of each short stick

Step 11:
This picture shows how one of the short sticks is being glued to the bottom part.

Step 14:
Finally eight plugs are made to close the openings.

Step 13:
Place the top part of the aquarium on top of the four short sticks

Step 19:
Ribbon is used to hang the aquarium (and shark)

Step 18:
Insert the eight plugs into the open parts of the aquarium

Step 17:
This picture shows the bottom and top of each plug.