Hey Clint and coues,
Nematodes are completely random, sometimes they hitch hike on plants or the like and get in the water. They are totally harmless, and best way to remove is to add fish. When you do, do a full filter cleaning to remove any excess and periodically use airline tubing to remove dead matter from the substrate and plants during a water change.
P.S. if you've found these techniques valuable, help share the information with new comers by linking back here in your own journals when you use my techniques!