Is it one big long one like an inch or two, or is it a bunch of small ones like a centimeter that actually swim. I have had both. The long one appeared in new gravel a few weeks after adding it. I took it out and never saw another one. The little white swimmers are constantly in my tanks in varying numbers. Sometimes there are barely a few of them and other times they are a swarm. It is said that their numbers are proportional to the amount of feeding. Also they are harmless. After cleaning a tank, I kept the water to see what comes of them. I can't say it is a concrete link, but the bucket developed blood worms and then I had a swarm of midges. The experiment is worthy of repeating, but I am not so certain of going as far as swarming midges.
Currently I am dealing with red worms (nematodes) that are hanging from two fishes vents. Nothing available to us at the lfs's has worked but I have tracked down some stuff that should do the trick. It is hog wormer that is safe for the fish. I have been treating my fish for almost twelve hours. They seem a tad stressed, but nothing as bad as other fish meds. The treatment takes 48 hours so the fishies have a while to go.