If you have changed the rear cassette and did not replace the chain, it will skip. It could also be out of alignment. There is a barrel screw where the cable goes into the rear derailleur. It adjusts the derailleur to the cassette laterally. It may need to be adjusted. If you've had an accident lately, it could be that your derailleur hanger is out of whack. Prolly nothing to do with chain tension. That's all I can think of right now.