I've never had any issues with them harming anything other than aiptasia. They WILL however, walk indiscriminately all over wherever/whatever they please. You'll be admiring how much your zoas have grown, and how healthy they look... Then BAM! Peppermint shrimp foot, smashed in their face. Candy Canes finally opening up after a rough few days... Bam! Peppermint foot sandwich for breakfast. The only other issue I've ever had with peppermints is having a few disappear... and I mean, like totally disappear. No exoskeleton floating around, no fish big enough to eat them (or anything else for that matter); not laying on the floor like a dried snack. Just disappear. Other than all that, completely harmless in my experience.