To tell you the truth I have never tried it. I tried to use old cubes that I broke up and put in the solo cups but it didn't work. I think if I used new powdered Rockwell it might have work. The reason I use grow rocks is because I am a recycling freak and used Rockwell is a big problem, especially in Europe where it is used a lot.
Grow rocks you wash with a ten percent bleach solution and use over and over again.