Floating soap bubbles
The hypothesis that soap bubbles will stay afloat for a longer period of time in carbon dioxide than in air, is proven to be true.
Blowing soap bubbles is a form of entertainment for children, adults and yes, even animals. It is a very inexpensive way to spend quality time with kids. The soap bubble toy industry is also growing. Fancy battery operated devices are used to produce larger bubbles at a much faster rate. New soap formulae are also being created for the bubbles to last longer.
Repeat the above experiment again and observe the changes in color, size and shape of the bubbles inside the tank.
Try to compare the results by repeating the experiment using helium gas instead of carbon dioxide.
Consider repeating this experiment by using dry ice placed in a large wok, to create a layer of carbon dioxide gas on top of which the bubbles will float.
Soap bubble - http://en.wikipedia.org/wiki/Soap_bubble
Soap bubble - http://scifun.chem.wisc.edu/HomeExpts/SOAPBUBL.html
Handle the glass tanks carefully.