Secchi disks are the international standard for measuring phytoplankton abundance in the absence of turbidity. Simply stated it tells you when to fertilize. A good fertilization program requires precise applications based on current visibility. A good measurement is important to maintain consistent blooms and determine proper fertilizer applications. Normally the desired range is 18-24 inches.
Secchi disks are made of thick PVC and painted. They have an eye bolt holding the disc and a rope for measuring visibility up to three feet. You can add longer rope if you desire.