Also, if you have difficulty getting the anode rod back in, particularly a new one with perfect threads, make sure the threads on the water heater are clean. A wire brush can help with that, but if you still have problems, a thread tap will do it for sure. Since you don't actually want to make new threads, and instead just clean the existing ones, you'll want to be sure you get it engaged in the existing threads, and at least screw it in part of the way with your hands (instead of a wrench).
I have used this one when I actually had to re-thread mine (not just clean it) -- it's 3/4"-14 NPT:
https://www.amazon.com/gp/product/B00FXKW4F6
Dave