no need for a tensioner - I tried it and it creates too much drag. Just measure the final belt size and space the wheels.
What will work is a axel tensioner, but it really is overkill and with properly spaced wheels, the treads don't have a tendancy to slip off.
It is my experience that if you have a problem with tracks in general it will be that they are too hard "stretched".
Stretch till it feels firm & tight, fasten it, and it will naturally get a little loser, done :)