Spirulina Density Growth
Visualizing the growth of tiny spirals can be difficult because we cannot see their growth with the naked eye. The thing we can notice is the change in color tone as the culture becomes more dense.
A new culture split will have a light hue of green as there are a little bit of spiral cells in the medium. As the Spirulina splits and regrows using the sun and nutrients in the water the culture will get darker.
Over time you will be able to tell when a culture is ready to split or harvest, but at the beginning you will need a tool to help testing destiny. The tool used to measure density in a culture is called a secchi tool.