Since you didn't mention bottom drain I'm assuming there is some buildup of muck on the bottom?

Draining is an option but not one I enjoy.

Another option is scooping or vacuuming.

Of course the pond could still smell bad to you. Different people, different noses.

[sarcasm on]
I don't understand, I though bacterial pond clarifier, eco fix and such products kept ponds clean?
[sarcasm off]

The smell of rotten eggs is a very poisonous gas. It is caused by bacterial growing without Oxygen. It can only grow in areas that pond water with Oxygen can't get to such as in muck. But before I am going to try and figure out where it is coming from I want more information. Give us more details about the old pond and how it was expanded. Does this pond have rock and gravel on the bottom? Was the old pond clean when expanded? What is the size of the pond? How much water is moving per hour? Do the pumps run 24 hours a day?

Mike

Indeed toxic in high enough levels and not optimal. True for almost anything. Many fish have evolved in such environments. No need to panic.

Yes it is toxic. Extremely toxic! It will kill a person at levels in the air of .005 ppm. Are you going to get those levels coming off a pond in the open air? No. But your fish could see those levels if the water is not circulating. Yes there are fish and other creatures that can live at those levels but any fish that you would have a pond will not. HS poisoning is the only time in my life that I had to perform CPR and that person died three times before they could could bring him back to life for good.
Mike

If the bottom is not covered with rocks, other maintenance is adequate and you are testing the water I would guess you have something organic like a raccoon or squirrel, or even a bird dead in the pond. After only a month that seems the most likely problem. By this time feathers, hair and fatty particles would be showing up in the filters. If you find such a thing in the pond, the water and filters will still stink until bacterial balance shifts from anaerobic to aerobic.

Water changes and clean filters will make the process faster. Scoop and vacuum all debris. Check under the plants. They could hide the problem. All the additives are for special circumstances. Do you really need them?

I'm not going near a pond ever again. Thanks

I think we are confusing odors here. A putrid smell is unpleasant but distinct from the rotten egg smell. I think sleepless's solution is correct in either case, but if it is rotten egg smell, you're not likely to come up with a handfull of 'dead skunk in the middle of the pond'.