we were so worried about these things at first, too. but it became a headache after a while. Trying to get each one out, was a pain. But never noticed anything bad ahppening to our fish, just to me the wife. I moved a rock and and got stung by one on my finger. at first it felt like a piece of tape. Then i realized what is was, shortly after the pain came in. some stayed in my finger for about a week and a half. eventually they were gone. Just be careful next time u move any rocks.