Hello, you have one of two things going on. Either the float is too high and the water is running into the overflow tube. If so, then their is a screw to adjust the water level. However, since yours is running about only every 15 min., I would replace the flapper in the toilet tank. This is a very inexpensive fix and easy to do. Take off the old rubber flapper and replace with a new one, no tools required.
