Have I killed our mystery snail?

[Tarindak]

Our 8.5 gallon tank contains one Betta and one mystery snail. Despite weekly water changes in a major battle with algae, I detected the symptoms of fin rot in the betta and picked up some medications. I tried to make sure on the package directions and some quick googling that nothing I bought was toxic to invertebrates but I was also panicking a little about the betta being sick. After cleaning the tank and changing the water, I added 1.5 T aquarium salt and the first dose of erythromycin. Like an hour or so later I get nervous and google mystery snails and aquarium salt and find a result suggesting that minimal use (1 T per ten gal) is ok but otherwise is dangerous. I’m just feeling terrible over this. The snail is shelled up right now so I can’t tell what their status is. Because the packet of erythromycin was enough to dose a slightly larger tank, I just did another 25% water change to try to dilute the salinity but fear I may have been too late. Should I remove the snail completely? I had wanted to get a quarantine tank set up but never got it done and now I feel awful.

[Tarindak]

Actually since the water change I can see movement inside the shell and some slight movement of the whole shell itself so there’s still signs of life, but I still worry I compromised him.

Edited June 25 by Tarindak