Water Plants will absorb salts, nitrates, and dissolved nutrients of decaying organic matter in the pond. To create a natural balance and maintain clear water, try to have some from each of the four types:
Oxygenating Plants - Cabomba, Elodea, Hornwort, Sagittaria, and Vallisneria grow under water just like the plants you see growing in naturally occurring ponds and lakes. They absorb excess nutrients from the water that cause algae. They also liberate oxygen, provide spawning space for fish, as well as shelter and protection for baby fish. Use one bunch per 2-3 square feet of open surface area (less will be required for small ponds).
Floating Plants - Water Hyacinth, Water Lettuce, Duckweed, Salvinia, and the floating leaves of Water Lilies help to shade the water thereby reducing evaporation so that you won't need to continually add more water. At the same time, the water stays cooler creating a healthier environment for plants and fish.
Marginal Plants - remove a lot of the nutrients that algae would normally feed on. Consider hardy Pickerel Rush, Variegated Sweet Flag, Yellow Water Iris, Corkscrew Rush, Cattail, or tropicals such as Canna, Taro, and Papyrus to make the edge of your water garden more attractive and help to keep the water clear.
Water Lilies - available in two forms; hardy which can be cut back and left in your water garden over the winter, and tropical which you would treat as annuals or lift and winter inside. The significant visual difference between them is that hardy Water Lilies have their bloom on the surface of the water with the foliage while tropical blossoms rise on long stems well above the leaves. Both are easy to grow - they require only 6 hours of sunlight per day, rich soil, and calm water. Hardy Water Lilies bloom yellow, red, apricot, white, and pink. Tropicals are white, pink, red, yellow, purple-violet, and blue.
Snails - a beneficial addition to your eco-system. Snails feed on the algae that sticks to the side of your pond and submerged plant containers.
Fish - will eat some algae, mosquitoes and other insect larvae. Be sure you have the right number of fish appropriate for your pond size. Stock 2.5 cm of mature fish per one square foot of pond surface area. Do not over feed your fish as the algae thrive on decomposing fish food. Feed fish only what they can consume within the first few minutes.