Water Lettuce, known as Pistia stratiotes, the lettuce has been studied and shown that it is capable of removing nitrate, nitrite, ammonia, phosphorus, fecal matter, mosquito larvae, from ponds. The rate of removal is not as great as it is for duckweed, but still impressive.
These floating pond plants are light green, resembling lettuce plants growing in a garden, thus the name. Growing well in full sun or partial shade, the plants provide shade for fish, and filter the water. Considered pests south of the Mason Dixon line, and is just as prolific as is the Water Hyacinth in warmer climes. Unlike the Northern half of the world, these plants do not expire in the winter months, and grow to the point of choking water ways. For this reason, it is outlawed in some of the Southern states.
Using up pond nutrients, this plant is generally suited to a larger pond, likes to be crowded, and fish love to eat it. Placing the lettuce in a floating basket or partition from the fish will serve two purposes. The fish cannot eat them and they will not be able to overtake your pond.
Over wintering this lettuce is a hassle and because they are relatively inexpensive, not worth your time and trouble. Just purchase new plants in the spring, not summer. If you do received them in hot weather, you may only get a head of lettuce with no roots. To revive it, place in container of water in shade to promote root growth, add some fish emulsion, but never in your pond(this is why the new lettuce should be in a container until it roots).
The end result is that water lettuce is another floating plant that can enhance your pond's ecology if employed carefully.