There already is salt in the tank at that amount as I was using it to treat Ich along with Ich-x
The rainbowfish died this morning, so I think it is Columnaris.
Can I use Melafix and Pimafix to treat it?
Colin will deal with treatment, but I will just say, never use more than one medication/chemical in a tank as they may interact and make things worse, including the death of the fish. Stay with the selected treatment and do a major water change (may involve good substrate vacuum, sometimes may have to include new filter media, depending) before using a different medication.