How does this happen?

[DepotFish]

I'm just finishing up a round of maracyn plus, melafix, and salt treatment in a tank with cloudy eye, columnaris, and ich. Fish are doing fine, but this morning I noticed nematodes on the tank glass. They can survive all the treatments I've got in the tank?!? I've never had them before and I don't intend to keep them. One fell off the tank glass and landed on a fish, who then started scraping against thing to get it off his back. I don't overfeed, its a barebottom tank and I can tell you the tank bottom is completely clean within a few minutes of feeding. I just don't understand how the little bugs could survive with no food and with all the treatments. I have a free afternoon so I'm going to empty the tank and clean it out. Its only a ten gallon quarentine.