Water Lettuce is a floating plant from Tropical America. Its thick, fleshy, light green leaves are heavily ribbed and form a rosette that looks like a small head of lettuce.
Pistia is a genus of aquatic plant in the arum family, Araceae. The single species it comprises, Pistia stratiotes, is often called water cabbage, water lettuce, Nile cabbage, or shellflower. Its native distribution is uncertain, but probably pantropical; it was first discovered from the Nile near Lake Victoria in Africa. It is now present, either naturally or through human introduction, in nearly all tropical and subtropical fresh waterways. 
Common Name: Water Lettuce
Latin Name: Pistia stratiotes
Light: Does best with shade during the hottest part of the day, but can adapt to full sun.
Size: Individual rosettes can range from 2" to 18" across.
Being a floating plant, water lettuce obtains all its nutrients directly from the water. This makes it a great plant to use to combat algae. The long feathery roots also make great shelter for young fish and other wildlife.
Water lettuce is grown for its foliage, but it does bloom. The insignificant flowers are usually hidden down between the leaves.
Planting & Care
Water lettuce is generally care free, but there are a few reasons it may not thrive. Fish love to nibble at the roots and if they are large or there are a lot of them, they may kill the plant or even eat the entire thing. Placing the lettuce in floating baskets or partitioning them off from the fish with rocks may help.
Lettuce loves to be crowded and even though they are floating plants, they dont like to drift around. Corralling them with a hoop of plastic tubing, a hula-hoop, fishing line, or even rocks will keep them from moving around. Water lettuce also does not like to have water on its leaves.
Lack of nutrients will cause water lettuce to grow poorly and turn yellow. Lettuce is such a vigorous grower it sometimes uses up one or more nutrients in the pond. This can especially be a problem in very small ponds and ponds with very few fish.
There are two ways of fertilizing your lettuce. If you only have a few, you can float them in a bucket of water with fertilizer for a few hours at a time. If you have too many lettuce to remove from the pond, you can treat the entire pond with a solution of Muriate of Potash. Lettuce and other floating plants also usually respond well to any of the micro nutrient solutions specially made for pond plants.
Another problem can occur if you try to ship water lettuce during hot weather. You will end up with a head of lettuce and no roots. Float the plants in a shade spot and they usually grow new roots very quickly. Adding a little fish emulsion to the water (not in the pond!) may help, too.
Water Lettuce seems to prefer some shade in warmer climates, but is also susceptible to frost, and even a brief exposure to frigid temperatures produces scalded leaves that quickly turn to rotting areas
Uses
Water lettuce is often used in tropical aquariums to provide cover for fry and small fish. It is also helpful as it outcompetes algae for nutrients in the water, thereby preventing massive algal blooms.

Water Lettuce is a floating plant from Tropical America. Its thick, fleshy, light green leaves are heavily ribbed and form a rosette that looks like a small head of lettuce.
Pistia is a genus of aquatic plant in the arum family, Araceae. The single species it comprises, Pistia stratiotes, is often called water cabbage, water lettuce, Nile cabbage, or shellflower. Its native distribution is uncertain, but probably pantropical; it was first discovered from the Nile near Lake Victoria in Africa. It is now present, either naturally or through human introduction, in nearly all tropical and subtropical fresh waterways. 
Common Name: Water Lettuce
Latin Name: Pistia stratiotes
Light: Does best with shade during the hottest part of the day, but can adapt to full sun.
Size: Individual rosettes can range from 2" to 18" across.
Being a floating plant, water lettuce obtains all its nutrients directly from the water. This makes it a great plant to use to combat algae. The long feathery roots also make great shelter for young fish and other wildlife.
Water lettuce is grown for its foliage, but it does bloom. The insignificant flowers are usually hidden down between the leaves.
Planting & Care
Water lettuce is generally care free, but there are a few reasons it may not thrive. Fish love to nibble at the roots and if they are large or there are a lot of them, they may kill the plant or even eat the entire thing. Placing the lettuce in floating baskets or partitioning them off from the fish with rocks may help.
Lettuce loves to be crowded and even though they are floating plants, they dont like to drift around. Corralling them with a hoop of plastic tubing, a hula-hoop, fishing line, or even rocks will keep them from moving around. Water lettuce also does not like to have water on its leaves.
Lack of nutrients will cause water lettuce to grow poorly and turn yellow. Lettuce is such a vigorous grower it sometimes uses up one or more nutrients in the pond. This can especially be a problem in very small ponds and ponds with very few fish.
There are two ways of fertilizing your lettuce. If you only have a few, you can float them in a bucket of water with fertilizer for a few hours at a time. If you have too many lettuce to remove from the pond, you can treat the entire pond with a solution of Muriate of Potash. Lettuce and other floating plants also usually respond well to any of the micro nutrient solutions specially made for pond plants.
Another problem can occur if you try to ship water lettuce during hot weather. You will end up with a head of lettuce and no roots. Float the plants in a shade spot and they usually grow new roots very quickly. Adding a little fish emulsion to the water (not in the pond!) may help, too.
Water Lettuce seems to prefer some shade in warmer climates, but is also susceptible to frost, and even a brief exposure to frigid temperatures produces scalded leaves that quickly turn to rotting areas
Uses
Water lettuce is often used in tropical aquariums to provide cover for fry and small fish. It is also helpful as it outcompetes algae for nutrients in the water, thereby preventing massive algal blooms.