Both WaterMom and I, and many others have added DE and the results are excellent.
Record your pump's pressure (on high if a 2 spd or VS) apnd keep it running.
Add 1/4 cup of DE and wait 20 minutes.
Wait, oh 20 minutes and see if the pressure has increased by 1pound, if so you are done. If not, add another quarter cup and wait again repeat until you reach one pound increase in preassure.
If you go over more than two punds, just backwash it out and start again. Try 1/3 cup this time.
Once you find the right amount, which will be between 1/4 cup and one cup, just add that much everytime after you backwash.

That's all there is to it!