Pondkeepers with an understanding of filtration know the nitrification cycle and how plants play their role.
Fish naturally secrete ammonia into the pond water. Biological filtration works to convert the ammonia into nitrites then the nitrites into nitrates. The nitrates are then used by the plants. If there are not enough plants using up the nitrates in your pond, you end up with an algae bloom (in extreme cases of nitrate buildup the fish may suffer as well).
Plants that cover the pond surface also reduce algae by limiting the amount of sunlight reaching the water. For the best balance in a water garden around 2/3 of the pond surface should be covered with plant foliage.
Another, often overlooked, use of plants in the pond is protecting your fish from hungry predators. Whether it is tall plants at the water's edge that help prevent a raccoon from reaching the pond or the water lily pads covering the surface that give the fish a place to hide from a heron, a few plants may be all that is needed to avoid making a meal out of your pet fish.
Koi and Pond Plants
Koi can be very hard on your plants because of their persistent rooting habits. They will root around looking for insects and larvae to feed on. Any valuable plants should be planted in tubs, which are then placed in the pond.
Also make sure that if you are using bricks as a platform to set your plants on, to use some concrete pond paint on them to prevent them from leeching lye into the pond. This will be toxic to the fish and cause you to have extremely high pH levels.
HARDY WATER LILIES:
Lily pads rest on the water surface (when crowded some pads rise slightly above the water). They are attached to the rhizome via long stems. In hardy lilies these pads are mostly all green, some varieties have light mottling and new leaves of many hybrids emerge with a red color.
The flowers, which appear in summer, also float on the surface, some hybrids have flowers that will rise a few inches above the water. Flower colors in hardy lilies include white, yellow, pink, peach, red, and variations of each. Hardy lilies go dormant in the winter and return in the spring provided that the rhizome did not freeze.
Hardy lilies generally do best when placed with the crown of the plant around 12" below the water's surface.