Still having problems with duckweed C.J.? I've been building and learning about ponds for more than 40 years now so perhaps I may be of a little help. But first let me say this...
I do not subscribe to herbicides and do not EVER recomend them. They are just too harmful, as they cannot be contained.
So, duckweed. You know, duckweed is not a bad thing at all in moderation. It provides an inviroment for fish fry as well as all the little critters at the low end of the food chain. It also helps to control algea blooms. But if it becomes invasive, yes, it can be a problem. If you find your pond surface covered with duckweed, it is usually a sign of out of control levels of nitrates, sulphates, and phosphates in the pond. Which point to too much organic sludge at the bottom. In this case, the mat of duckweed can only exacerbate the problem by raising ammonia levels, which can lead to not only a fish kill, but a dead, stinking, cesspool.
So, how to prevent it first. Well it eventually may come down to the point that the pond has passed it's lifespan. Face it, there are ponds that are beyond fixing. Ponds that are surrounded by trees are bound to not be able to handle the many years of sludge building up from the years of autumn leaves. And without dredging, they will eventually become fond memories. But with that being said, let's address too much duckweed.
The first thing I recomend, is easy and quite inexpensive. Goldfish. Ot even amur, just plain old cheap goldfish. They will feed heavely on duckweed. They will grow large, and can be removed by netting or fishing, if needed. Ducks do eat duckweed, but it's not at the top of their menu.
Second, may be labor intensive, but removing duckweed by skimming is by far the fastest way. There are products made for this task, even automated skimmers, but I cannot comment on these, as I've no experience with them at all.
I suggest you do some web searching at this point. I know that there are also a number of products that add live enzymes and helpful bacteria to a pond, that actuallyfeed on the sludge build-up in older ponds. There are also some that feed on the duckweed itself.
If I were you, I'd hit Petsmart and buy a bunch of feeder goldfish first. They can tolerate being stocked into warm water at this time of year, as long as you let the water they come in warm up by floating the bags for a while first. You may have to pay more visits to the pet store, as they will soon be noticed and added to the diets of fish, raccoons, and raptors that notice them. And as they begin to chip away at the duckweed, you can research other non toxic remedies.
Good luck!