By experience with your pond. A good rule is to go slow if fertilization is a part of the plan. If not then you are talking about the mgt of natural fertility. Knowledge of what the variables are and how they all work and fit in the system is the best way to avoid problems. The three most common problem areas are excess fertility/nutrients (several sources including soil, fish feed and waste , runoff/fertilizer etc), timing/weather and water level/flow.