My experience is that unless they have a predator, they will multiply nearly uncontrollably. I suppose they would eventually run out of algae / detritus to eat and that would limit their population, but I never seemed to reach that equilibrium. At it's peak, there were nights when my front glass would have 2-3 / sq in. when I shined a flashlight after lights out. There were similar numbers crawling all over the rocks. I manually pulled a bunch out, but the numbers would be right back after a few weeks. That's when I got the Harlequin. Two months later, I struggled to find any at night, and the Harlequin doubled in size...In regards to the asterina stars I have are they considered the type to die off or shrink in population or will they just continue to spread and grow unless addressed.