Every Eco-system achieves a balance, an ecology can not be destroyed,  only changed so that the present life forms must find a new population balance, even if the poplation is zero.  I want my pond to achieve a balance so that the fish thrive.
Unlike a large lake, the pond is small, and is sensitive to small changes that a lake would easily tolerate. The inputs must balance the pond systems ability to absorb.
I want my pond to:

1. Have clear water
2. Support the Fish
3. Require as little maintenance as possible.

1. the Amount of light
2. the Amount of food fed to the fish and therefore their waste production
3. the population of the fish, amphiblians and reptiles.
4. the population of plants.

The amount of light is controlled by large plants such as the water lilies and the surrounding trees. When the amount of light to the water is low enough, the algae doesn't grow very well and the water doesn't turn very green. It also helps to introduce aquatic plants to compete for the nutrients that the algae needs. It is impossible to eliminate all of the algae from the water without using ultra-violet serializing lamp to kill the algae passing through the lamp. Bacteria break down the fish waste into less toxic chemicals until eventually the plants use the wastes for food.

The fish are fed very little food, I make them browse for the algae that grows on the sides of the pond liner. This isn't cruel because the algae is very nutritious for them, and fish are cold-blooded and don't need energy for heat like we do. More fish die from being over fed than have starved, watch for lively activity, and bright colors as signs of good health.(My fish are deeply colored, almost red!)

The wastes from the fish is broken down by the biological filter, which in turn feeds nutrients back to the plants and algae that the fish eat.