chain tensioner?

[Firsttimer]

so im just wondering if a chain tensioner is nessary and if anyone else has one. im running a 120 link chain, but i have a swingarm. i see alot of the the hardtails use them, but cant find a pic of a streched swingarm with one. any info helps. thanks

 

[drewpy]

how "floppy" is it?

..and how slack is the chain