There are a whole bunch of different species of marine Lyngba - that produce a couple dozen different toxins. 4 of those toxins are molluscicidal. Its a bit luck-of-the-draw with which actual species you have.I’m surprised you’ve had luck with snails with this stuff. The pacific Mexican turbos and then the zebra turbos from the Caribbean seem to go down quick once they ingest this stuff in my tank….which makes sense because it’s extremely toxic.
He says his snails are doing ok - so hopefully he's got one of the ones that isn't.