A Secchi Disc is a measuring tool that will help you to measure the density of your Spirulina (or any other microalgae). Traditionally used in large ponds, this Secchi disk is a hand-held tool suited for a small-scale setting such as aquariums or bottles. Check here if you’re looking for a larger Secchi Disk for your ponds. To measure your culture density, lower the disc so it is perpendicular to the water surface. When the white circle is no longer visible, stop and look at the number indicated on the waterline. The lower the number, the more dense your spirulina is. Harvest at 3mm or lower.