If you see one, the whole tank is infected. The med you bought has the right ingredients to kill the nematodes. If we stopped calling them worms, we'd stop using the wrong meds...
The odds your entire tank is infected are only about 99.999999%. If you see a Camallanus, that's because it's hanging out to spray tens of thousands of eggs into the water column. So the bigger the tank, the bigger the problem and cost.
Follow instructions, vacuum as you can and they should be eliminated. I have never had a comeback in 3 infestations over the years.
Camallanus sp bite into the intestinal wall, and as the med forces them to let go, internal bleeding can happen. You can easily lose fish in the days and weeks after a treatment. I first saw this parasite in the early 90s, and it killed everything in days. It's adjusted to its new hosts, the ones it's met on the fishfarms, and it now acts like other parasites. It's slower to kill, which enhances its ability to spread. It can operate as a chronic parasite for a while.
The med paralyzes them, but I have never had problems with regular vacuuming. You have to do more than one dose, as eggs are possibly in the fish, and stopping too soon gives them a chance.