PF

I recently lost 3 female bettas, 1 male betta, 1 african dwarf frog, 1 female guppy, 1 clown loach, and 1 cherry barb. This aquarium is months old. the fish were pretty old but not that old. my replacement female betta died as well. My aquarium is a 30 gallons. I checked ammonia, Ph, nitrate, and nitrite and everything is in safe status. The only thing that I did different this time when I last changed the water was added a bit more aquarium salt to kill off the snail. My temperature has also been at a steady 80.5-80.7. What's wrong? Is my salt killing my fish?
Oh, and the fish that seem to be doing fine and perfect are the following:

1 gold dojo
2 skirt tetras
1 bamboo shrimp
1 red minor tetra
and three dwarf plattys