I bought my first betta rescue last Sunday: a gorgeous little red VT boy who I named Jeremiah. He didn't have anything visibly wrong with him, but he was inactive in putrid water. 
Now he's developed fin rot on the edges of his tail: it's at the early stages, just black edges and splits about this long: --
He's acting fine, and I've put some Promethyasul (multi-purpose med) in his tank but to be on the safe side I'm going to add some salt today. I last did a water change on Saturday.
I have cooking salt, these are the ingredients:
'Salt, anti-caking agent.'
Would this be okay to use? How much for a 1.5 US gallon (6 litre) tank? When should I next to a water change?
Thanks.

Now he's developed fin rot on the edges of his tail: it's at the early stages, just black edges and splits about this long: --
He's acting fine, and I've put some Promethyasul (multi-purpose med) in his tank but to be on the safe side I'm going to add some salt today. I last did a water change on Saturday.
I have cooking salt, these are the ingredients:
'Salt, anti-caking agent.'
Would this be okay to use? How much for a 1.5 US gallon (6 litre) tank? When should I next to a water change?
Thanks.