Duckweed can be a nuisance but is not something to worry about. If you do get goldfish, they will eat it. The little red worms could be midge larvae. The little bugs could be anything. An outdoor pond will get all kinds of critters in no time even if everything you put in has been sterilized. Those of us who keep fish worry about introducing a parasite when introducing a new fish so a couple of weeks of quarantine is usually the way to prevent that.

Absolutely the plants will be contaminated with whatever was in the ponds the plants came from. Most nurseries don't keep Koi or even Goldfish in the ponds so threat of viruses should be low.

The most harmful things would be other plants like the duckweed, but also string algae and other macroalgae. This is both good and bad. No one likes string algae but it is the most likely reason for green water turning clear when plants are added. The plants themselves always get the credit, but it's what the plants bring with them.

If you're concerned you can place the plants in a holding pond, or better yet, several holding ponds. Wait a few weeks to see what grows. If you don't see duckweed in a holding pond you know those plants are probably clean.

Macroalgae is pretty much impossible to get rid of. You can plant marginals in regular, well watered, soil for a year. That might kill macroalgae, might not.

Things like duckweed can be kept out of a pond using this method. Macroalgae can't really be kept out of a pond using any method as it will blow in on the wind. So clean plants could keep macroalgae out for a couple of years, but you'd probably have green water in exchange. Some algae is going to fill that niche.

Another issue is birds coming to the pond. Waterfowl and fish eaters will bring everything the plants could bring in plus viruses and parasites in their waste. So really, the plants aren't normally a big concern.

I actually would welcome duckweed in my pond instead of this green pea soup algae - tried to establish some but fishies eat it up before it gets established. Also its very easily netted out. While you dont want your whole half acre pond blanketed in duckweed, a nice bunch it around the edges of the pond visually softens the edges and would look attractive in combination with the marginal plants.

I would try to learn more about what your ecosytem will eventually be - what exactly are the plants/insects that you want to be there, and which you dont want. What exactly were those bugs and worms - some invasive species you dont want or maybe beneficial things you do want? Frogs for example will need to have bugs to eat. When you say "sterilized" - hope you were being facetious because you really dont want that!

@ kashka kat: FINALLY got some duckweed established here, lol. It does multiply exponentially almost overnight but my fishies' appetites were greater. Now they just get it as an occasional treat:)

Having said that, I understand your concern, czecheart, with a 1/2 acre pond, especially one intended for swimming. Out of my league, experience-wise. Sorry, no help from this quarter. I can only say that all of my plants go in a holding tank for a time, then get a good rinse before being added to my pond. Never had a problem. As for the critters, if you build it, they will come:) I don't worry so much about worms/bugs, but I don't have a 1/2 acre pond either (envious here).

blessings,
~digger