Glue the inside of a shoe box (or similar cardboard box) with blue paper and cut out green paper "seaweed" and any other sea creature shapes. Cut out and colour (both sides!) some of the fish shapes. Punch holes in the top of the box, and pierce small holes in the fish and hang them from a cotton thread tied to a matchstick. Put sand or earth, and add any fishy objects you can find, to the "floor" of the aquarium.