Maintaining the proper tension is easy and the only tool needed is an Allen Wrench or Hex Key that came with the treadmill. This tool usually comes with your treadmill.
Standing next to the treadmill (not on it) turn it ON and increase the speed up to 6 miles per hour. Grab your hex key and go to the end of the treadmill. On each side there will be a couple of holes providing access to the idler roller adjustment bolts.
If the belt shifted to the left you gonna need to tight the left side, if the belt shifted to the right you are going to tight the right side. Using the hex key, turn idler roller screw clockwise 1/4 turn and watch the belt, it will start moving slightly to the opposite side of the treadmill. Repeat it until the belt is centered.
If you are not sure what are you doing - DON'T DO IT, you can damage your equipment. Call professionals. We are servicing all types of exercise fitness equipment in TRI-state area.