Chain tensioners?

[CGcloner229]

Sorry for spam posting, mind racing this morning. Any have experience with chain tensioners? When would you use one? Im having an issue right now after replacing front sprocket with 17t had to install new chain which i found out is a pain in the ass. Never had to mess with the chain to this extent before. Connected chain as tight as possible, when pulled tight with one hand only about 1 link of slack at most but when you connect and let go the slack lossens to the point chain will come off back sprocket when riding. Cant for the life of me get the rear axel bolt off to tighted the rear tire up. Its already starting to strip. Would consider throwing on a chain tension next day from amazon if they work with no problems. I kno it seems silly all i gota do is tighten the rear but this bolt is from satan himself