MM, I would not, because duckweed is not a normal part of their diet, actually they never eat it, so it may cause problems for them trying to digest it.
But anyone who feeds broccoli or romanie would know to blanch it in hot water before feeding it to the fish. The reason why you do this is to remove the starch like substance that all land plants have in order to maintain rigidity. In the sea however there is no need for the plants to be so rigid, and they therefore they do not contain this starchy element in their tissues.
What i suggest you do is scrap the boring FW tank and grow some proper macro's for your tang.