How has your water tested over the weeks since you've cycled and added fish? I'm trying to imagine if anything is stressing the fish. Either water issues or you got an infected fish from the LFS
The quarantine/hospital tank
may help you in the future as you add new fish and QT
them for 2 -3 weeks to ensure you don't introduce parasites. As for now, you've either got to treat the infected fish with copper - which will prevent you from having inverts in that tank in the future.
I'm sure someone will chime in and tell you more about treating the fish, but don't scrap the SW
thing just yet. It's a wonderful hobby when done right. Maybe starting over with more caution about adding new fish directly to your main tank might be a good thing to do.
Back to the QT
It's a very necessary thing to have in this hobby IMO