What is wrong with my snails?

[nathan212178]

I have had this problem as of late, whenever I would put snails(asterina, Mexican turbo) they would move around for a bit at first but would then not move and just slowly become weaker to the point where the crabs get them. It is a macro display and the only ones that I’ve had success with are bumblebee snails, hermit crabs, and the recent sexy shrimp I added two days ago.
I added a small amount of panacure(dog medicine to get rid of hydroids) about a year ago, but would that still be in effect now? Should I just ditch the rock and sand and start over? Or is there another element at play?

 

[nathan212178]

I forgot but here are my levels I got from my lfs, he said nothing would harm snails( I fixed the salinity and did a small water change) the nitrate and phos levels are from my neonitrate and neophos additives for the macros