It has been my experience that zoas can be a bit finicky when it comes to placement. This again, from my experience, will affect growth. I have a colony that started with 3 polyps and has turned into well over 80 in less than 1 year and it is starting to cover the bottom of my tank. I also have rastas that stayed with 3 polyps until I moved them to the very top of my tank. I also have some emeralds on fire that no matter how high I put them, have a relatively slow growth rate.