Salt- how much is enough

Hi. I have a 120 litre tank with 1 green severum 1 female betta, 2 bronze
corys, 1 bristlenose.
Things were going well until I purchased the severum. I was floating it in
it's bag in the tank when I realised that things weren't right in there.
Both bettas looked sick (scales seem to be dropping off, 1 has since died) 1
of the cories looked like it had a fungus infection. In a panic I checked
the water specs. ammonia. nil, nil nitrates, But PH was low.
also GH was a bit on hard side but KH was really low.

Anyways as I had no other Tank for the severum he had to go in. Added some
KH raise and corrected ph. I added a broad spectrum type med.
I have heard that salt can be beneficial. As I have never added salt to my
tank before I was just going to follow the instructions that came with the
"sea salt for freshwater". The rate given was 1 gram per litre... which when
weighed out seems a tremendous amount of it.
1. does this sound right ?
2. Will the broad spectrum med treat the ailments mentioned above, or
should I be getting something more specific ?
3. is there a readily available test for salinity in freshwater aquariums ?

thanks in advance.