The above is a diatom. The rectangle shape is just one of the many that this little creature can take. This particular algae can be found in freshwater sources.
This is the first time I have viewed a desmid in my aquarium. Though they can take many forms, the above is a popular one found in East Tennessee.
Wolle, Francis. Desmids of the United States and List of American Pediastrums. Bethelehm: Moravian Publication Office, 1884. Print
I noticed an increase in the activity of my aquarium after the pellet was added.