Submerged aquatic plants grow completely under water and are very important to the ecology of your pond.
They have a significant effect on water quality, provide long term nutrient storage helping to keep planktonic and filamentous algae in check, and oxygenating the water.
These plants provide shelter for your fish and produce large quantities of invertebrates used as food for the fish.
A good balance for the pond's surface is between 15 and 20% coverage. Providing food for waterfowl, such as certain duck and grebe species, in the form of the invertebrates and the plant as well, egrets and herons also often hunt the shallow areas of a pond with submerged plants.
Be aware of the egret and heron, for both are fish predators.
To help control the predators, use pond netting to keep your prized fish safe. Examples of submerged plants include fishweed, known as anacharis that can either root or float free in the pond, tape grass with bright green leaves that grow completely under water,and Red Ludwigia Bunch with its red-green leaves that add a bit of color to your plant selection.
The plants can also be potted but remember to set them at least 12" deep in order to reap the benefits and keep the plants viable.
Worldwide populations of submerged aquatic vegetation have dwindled to the point that there is a large scale restoration project to improve the ecology of our waterways, especially as it relates to fish habitats.
Shallow lakes, such as Lake Waikare in New Zealand are troubled and in poor condition due to the heavy use of fertizers by the farming industry.
To allay further deterioration of the waters, experimentation with water levels is underway to try to increase the light and the submerged vegetation.
In conclusion, if you plant submerged aquatics, you are not only doing yourself and your pond a favor but in some small way contributing to the overall ecological balance of nature that this planet needs to survive.