overheats

Was the overheating occuring before you replaced the water pump? Did you purge the air from the cooling system after replacing the pump? You can have an air pocket in the system causing it to push fluid out of the reservoir at temperature. Your running issues with this overheating has me very concerned - if you try to drive with this problem, you could cause major engine damage potentially. Resolve the overheating beforfe continuing to operate your car. Expensive damage could have already occured. The tensioner pulley should indeed allow the belt to be pulled up if you pull on the belt with force. The belt and pulley would not be your issue.