They are nematodes, completely harmless. They can "hitchhike" on anything that is introduced into your tank. They seem to literally appear out of thin air. Finding a tank that does not have them would be impossible. It just comes down to keeping the population in check(you may not see them but they are there) by not overfeeding, maintaining water quality, and having either "predators" that will knock their numbers down or other fauna(shrimp/snails etc..) that will out compete them for food.