Fertilizer is added to fish ponds to provide nutrients for phytoplankton. If your pond is full of weeds, adding fertilizer will only make the weeds grow faster. Once weeds are established they steal nutrients from the phytoplankton. Weeds also shade the water surface and prevent sunlight penetration which is essential for phytoplankton growth. Remove weeds before fertilizing.
Weeds use nutrients intended for phytoplankton.
People not familiar with fish culture often think fish should be cultured in constantly flowing water. Flushing water through a pond may help remedy situations where fish are under stress or appear sick, but this action can also flush fertilizers and nutrients out of a pond. This inhibits phytoplankton growth. To avoid this do not allow a continuous flow of water through the pond. Add only enough water to replace evaporation and seepage, or correct problems.