The tension should not be too loose or tight. The tension of the bike chain is right when you can wiggle it 1cm up and down. 

Check our YouTube channel on how to adjust the bike chain, or visit a local bike shop to make sure the tension is always optimal. 

Your chain must be fitted correctly, especially if you have a bike with only coaster brakes/backpedal brakes. Any chain malfunction can cause potential brake failure and subsequent accidents.