You can treat in the 5g. Yes, you add 5 tsp of salt, predissolved and very slowly over an hour. You only add more salt along with water changes. The salt will stay in the tank until you change the water.
In a 5g you don't need to do daily change. You could do a 25-50% change every other day with 100% at week's end if you wanted. I think 50% mid week and 100% week's end would be plenty, personally. Follow instructions for dosing Maracyn per the box.
Maracyn is Erythromycin. It has some potential to work but not at the same rate as the Kanaplex (Kanamycin). You could try it, but in addition to treating I would order the Kanaplex just in case it doesn't work. I have also seen it suggested to use Erythromycin as a double dose, but I don't have any experience with this.