I was doing a water change in one of my tanks and I saw quite a few little wiggly worms swimming through the water. They were a brownish color and swam with an S motion. What are these things? They are creeping me out.
Yes creepy, but not a huge concern. They're either planaria or nematodes and if you have them its a sign that you are either over feeding your fish, or have your tank overstocked--or both- thereby producing too much waste, (food in worm-talk). And it's also an indication that you're not doing frequent enough partial water changes.
For the next several weeks do partial water change of 30% three times a week. Siphon the gravel. If your gravel is more than three inches deep consider removing some of it after it's been thoroughly siphoned. Increasing water movement in the tank will also help by lifting the waste so that the filter can pick it up.
From now on make sure you're not overfeeding. One small feeding a day is plenty. And weekly partial water changes and gravel siphonings will also help.