I had the same problem. I had a rock mix of 3 colors and I only wanted one. The rock was removable, so it made it easy. What I did was take the rock out and took tweezers and razor blades and plucked away the ones I didn't want. I did a few at a time so that the zoas were not out of water for too long. It also gives you a chance to look at the rock after the zoas open and see which ones still need removing. After a few tries, they should all be gone.