Are there bad snails in a reef tank?

Bad algae, limpets are so cool.

 

but to answer your original question as well, yes, there are bad snails. I have heard of some that eat zoas, and seen it myself

 

Hi, I'm here, lol,

Yes, it's a keyhole limpet, which isn't exactly the same as the true limpets, all of which are algae eaters. Most of the ones that look like little grey mountains are the Indo-Pacific Diodora species. They do graze algae of different kinds off the rocks and glass, and may also eat sponges, although probably not any ornamental sponges that you find exposed in your tank (more likely the hidden ones growing under your rocks). I suspect that would include coralline, but maybe not enough that you'd notice. Their radulae are hard enough to scratch acrylic, although I don't know how often that really happens. It's possible, though.

Most people don't find the ones like yours objectionable at all.

Cheers,



Don