I don't understand all the hostility on such a simple question.
Here I'll try to help out because I am interested in such things.
Based on the picture and behavior you described, but it looks to me to be a water beetle larvae of some kind.
If you still have it, you can bring it up to Maryville College and we will look at it for you.... I'm still an amateur aquatic entomologist but under a microscope I could probably figure it out more definitively.