I need a little help here. I have a few pieces of $5 rock zoas, those cheap kind. Notice they open big and nice for quite a few weeks, then it started to close up and refuse to open. After 2-3 weeks staying close, these zoas will wither and waste away, not all, but most of the polyp on the rock. What could be the problem? Those zoas that cost over $20-30 are doing fine and show no such sign. I'm puzzled, hope someone can shed some light. Thanks.