In the past 5 years, I have seen zoas comes and go. I once talked to a old timers and he told me why he did not like to keep zoas especially those expensive ones. He said zoas had limited life span and they did die away. For example, i have a large pink zoas colony for over 4 years and almost none of the polyps is the original. The rock where the original polys stayed is almost empty now.
The bottom line is that if you can not get the zoas multiply fast enough, you will lost them. That is why people should not buy zoas by polyp. You need to have at least 10 polyps colony to start with if you expect to keep that colony for a long time.
SPS is entirely another story. They live forever.