New HF dual drum rock tumblers a rollin'

Just picked one up today along with some aquarium gravel, got it rollin now. The dual drum was only $10 more, plus I payed an additional 10 for a 2 year replacement warranty if anything happens. Had a 20% off coupon which dropped the price to $40. I checked the belt, seemed to be on good, but there is quite some slack in it, which could be why alot of people were experiencing breakage issues. It came with 5 replacement belts. Is there anyway to adjust the tension in the belt? First 30 minutes of tumblin and the belt is starting to squeel.