Now’s the time to fertilize ponds. Use only brands with a high phosphorus number that is approved for aquatic use. Regularly measure plankton density? Preferred visibility is 18 to 24-inches. If you’re pond develops a “pea soup” color during the summer and visibility less than 14-inches, call a biologist. You may need immediate aeration. Fertilization increases productivity of a pond, but nutrient levels must be monitored closely to maintain healthy balances.