Was just a joke, but seriously if it took a microscope to see them, it probably wasn't roundworms. Just about any wild garter is going to have nematodes. Many of them are harmless. Still, treating the snake with panacur is a good idea. it can get rid of a wide range of parasites, including a lot of harmful ones. It just doesn't seem to work for certain tapeworms. (the kind that use fish or amphibians as an intermediate host) For that I use praziquantel and repeat the treatment two weeks later. But if your snake has those you would know it. They have poor appetite,(or "can't get enough" to eat) low body mass, malnutrition, sometimes blood in the feces, and segments of worms coming out of them. It's pretty obvious.