One of the first upgrades you can do on top of a vanilla Prusa i3 is to add an X axis belt tensioner.
I designed this X axis belt tensioner for a Prusa i3 variant - ANet A8. But it may very well work for other Prusa i3 variants.
You need to print the parts with support on. Feel free to make suggestions on how it can be revised to eliminate the need for support.
The design is available at OnShape here so that anyone can customize it to their own needs.