Zoas can shrink for a number of reasons. Something pestering them, too much light, too much flow, or not enough food. And then there are the Hawaiian PE morphs........they are incredibly finicky, and a lot of folks have an impossible time keeping them. I'm having fits with mine right now, and they may, or may not, make it.

And honestly, sometimes you can do everything just right, and they still won't survive. It's maddening.