You need submerged vegetation

If you don't have anacharis, cabomba, or one of the submerged plants that gives off oxygen you cannot have a balanced ecosystem. Most of the letters I get ask why fish die, why water turns green, what makes the pond turn into a cesspool. The best answer I have is that there is no submerged vegetation. The fish eat it, fish waste feeds it. It serves as the very lifeblood of the pond.