Chlorella are free living, single celled, non-motile green algae which can live in a commensal relationship within tissues or cells of invertebrates (eg. protozoa, hydra). Chlorella are often found in fouled or untended aquariums, and frequently seen in mixed algae cultures derived from pond water. The organism has been extensively studied in connection with photosynthesis, and for its potential as a food source.
Fresh water sponges are invariably green because of Chlorella. The size, shape and number of pyrenoids are used to differentiate species, and there is a thin cup-shaped or platelike chloroplast within every cell.
You will need to allow 4-8 weeks to grow the Chlorella into a more concentrated and larger volume that can be harvested for mixing with sodium alginate to make algal balls.
For optimum algal growth, use Chlorella culture medium, It is also possible to use water obtained from a healthy aquarium or pond as your growth medium, but there are risks that you'll grow other species when you use water from another source. Distilled or demineralised water is not suitable as it lacks the nutrients the algae needs to grow. Tap water is also problematic because it not only lacks nutrients for the algae but may also contain residual amounts of water treatment chemicals that do not favour algal growth.