Glue construction paper to the jar lid to cover, if desired.
Wrap 1 pipe cleaner around the pencil. Remove the pencil. Stretch out the coils to make wavy seaweed. Cut the coiled pipe cleaner into several pieces. Bend 1/2-inch foot onto each piece of seaweed. Cluster several weeds together at the foot and secure to the inside bottom of the jar with glue or tape.
Pour the cereal into the jar to a depth of about 1 inch. Arrange shells and/or rocks in the cereal.
Thread a length of fishing line through the darning needle. Use the needle to thread each fish onto a piece of fishing line. Hold the fish up to the bowl or the jar to determine the length of the line. Tape each line to the inside of the lid of jar.
Lower the fish into the jar and screw on the lid.