It is time to fertilize again if the disk visibility is increasing rapidly toward 24 inches or if the disk is visible past 24 inches. If the disk disappears in less than 12 inches, the bloom is very dense and a severe oxygen depletion could occur. A Secchi disk reading of 12 inches or less means the pond is too nutrient rich. At that point you need to determine where excess nutrients are coming from. Try to discover the source of the problem.
Dense blooms can consume most of the pond's oxygen at night.
Be prepared to aerate at night if the visibility is low and there are consecutive days of cloudy weather. A secchi disk can be used to determine when to fertilize. Granular or liquid fertilizers can be used in ponds. Granular fertilizers must not be broadcast into the pond. Granules will sink to the bottom, and the phosphate will be absorbed directly into the mud and be lost.