I cleaned out my 5 gallon, and just before dumping the water out, I noticed hundreds of these little thread-like white worms swimming around in S shapes. I was thinking of dumping a few in my fry tank, but then thought the better of it and just flushed them down the sink.

I treated the gravel by soaking it in a high concentration of salt water, since I tried it in a glass of water, and the little worms started jerking around, then finally stopped moving and sank to the bottom. Then I rinsed the gravel a cup at a time.

I think they're all gone now. I probably ruined the cycle because I washed EVERYTHING. I don't know where those worms came from, but I don't want them coming back.

The white worms are planaria and are usually found in most systems.
If you start seeing them then you need to up the changing routine and cut back on feeding as a lot of them shows a dirty tank and over fed fish.

I just googled planaria and they don't look anything like what was in the tank. Mine were as thin as cat fur, and about 2 millimeters long, white, and they snaked around in smooth S shapes.

My bad, they do appear to be planaria. I found a website that says small fish eat them. I guess I could have caught a few and given them to the fry.

Would they appear if I filled a cup with 1/2" of gravel and "Fed" it everyday?