The fish weren't healthy when you got them.
PetSmart fish often aren't healthy. I got these two scrawny weaklings in quarantine right now and am not sure how they will fare. They are from PetSmart as well. Compare them with the Asian shop fish. Big box store fish are often full of bacteria, parasites, and other diseases when you get them. Some make it, some don't.
I suspect this has little to do with your water quality or their environment , and a TON to do with the fish themselves. You may not be able to save any of them. It does look bacterial in nature. Salt will probably not cure them - it looks pretty advanced. I would be reaching for something stronger.
Plants and all that will not help them now - there is disease spreading and it won't magically reverse just because you put some plants in.