acro's will normally grow to a rock if you can keep it in place within a week or 2. rubberized cyanoacrylate is my favorite glue that is black and blends into your rock better than green or white. acro's in the wild grow as much as 10 cm per year according to books I have read. I would only expect this under the best conditions. I have seen this much growth in the past out of only a few in my own tank.