how to get rid of duckweed.

What is the little green plant floating on my pond?

It's probably Duckweed, a tiny single free-floating aquatic perennial weed that grows in nutrient-rich waters and is common in garden ponds.

How to Identify Duckweed:

 

 It is an easily recognizable tiny light green free-floating plant with 1 to 3 leaves 1/16 to 1/8 inch in length. A single root (or root-hair) protrudes from each leave. It tends to grow in dense colonies and in still water quickly covers the entire surface of the water depleting oxygen and cutting out the sunlight that submerged plants need for their growthIf left uncontrolled it suffocates and kills the pond.

How does duckweed spread so quickly?
 
In Summer it reproduces by growing  two or three tiny baby plants that bud from the original, these seedlings then reproduce new plants in the same way. In a nutrient rich pond, the plant mass can double every few days, quickly covering the surface of a garden pond.
 
How to prevent duckweed getting into my pond.
 
The introduction of duckweed into garden ponds is almost impossible to stop, it can be brought in with new water plants. Duckweed can also be brought in on the feet of  birds even frogs visiting from a near neighbours pond. The only way to stop it suffocating the pond is to control it.
Its got single round leaf with a small root below
single round leaf with a small root below

How to Control duckweed without using chemicals

Removal
  • Duckweed in small ponds can be kept under control by continuous netting. 
  • For larger ponds, using a floating boom to sweep from end to end and then removing the mass with a large strong net works.
  • Duckweed makes great compost, providing its composted away from the pond.
  • Planting Waterlilies and other plants with floating leaves will also reduce the level of duckweed.
  • Waterfalls and moving water, will push the weed to one end of the pond making it easier to remove from the pond.
 
Reduce nutrients in the pond. 
  • Duckweed needs nutrient-rich waters to thrive, the single most important thing you can do to keep duckweed to a minimum is to: 
  • Stop over feeding your fish. 
  • Stop adding fertilisers for pond plants, they don't need it, fish, frogs, newts all pooh in your pond, providing all the fertiliser your pond plants need to keep them healthy.
  • Unless its a newly built pond and the substrate needs to build up, removing leaves before they rot and sink to the bottom will also help reduce surplus nutrients.

Is Duckweed a problem for my garden pond

What's the problem with duckweed? It's not a problem if kept under control, it provides shade, some fish eat it, pond insects love it and it's not unattractive floating on the surface of a nature pond. 
Is duckweed good for wildlife ponds?
As I have said above, if controlled it can be beneficial to a wildlife pond, but uncontrolled duckweed is a problem, if the surface is completely covered sunlight is shaded from getting into the pond. All living things need light to survive, so totally shading out sunlight will kill the submerged plants, invertebrates and other pond-life living in the bottom of the pond. They would die and decompose using up any oxygen that  was getting through the duckweed. Eventually Killing off the ecosystem, leading to the death of the pond. Meanwhile the Duck Weed would continue to grow.
I have seen this happen in wild ponds, leaving only the duckweed and marginal plants living and of course the ducks.
Controlling Duckweed with chemicals and bacteria.
There aren't any chemical Duckweed-killers available to the public. Some garden and water garden retailers stock a product called 'Duckweed Control' it contains a bacterial culture that removes nutrients from the water to discourage duckweed. They say it's safe to use in garden ponds and won't harm any of the animals including newts and frogs at various stages of development, plus stacks of invertebrates. 
On the label it says "eco friendly" and "frog friendly" and "contains selected bacteria" and "suitable for use in all types of ponds". It doesn't have a list of active ingredients, though it does say "wash hands after use" and "keep out of reach of children".
I would be wary of damaging the eco-system  in my 'wildlife' ponds. I prefer instead to control the duckweed By reducing the nutrients naturally as I have explained above.