Duckweed is fine for aquariums, but I wouldn't get it from a pond as you have no idea as to what the water quality is like in there, what diseases there may be and any pests or eggs of aquatic insects which may harm your fish.
You could wash it I guess, but I wouldn't risk it.
If your tank has a light, you could try some easy to grow low light plants like Elodia and crypts. Moss balls and Java moss will be fine as well.
If you don't fancy trying live plants then silk will be fine but yu do need at least another 5 I'd say. Look on ebay and see if you can get a pack of them cheaply.