Salt Water Treatment for Goldfish?

I have a 16 gallon tank. One black moor, about an inch long, with cloudy eyes and a fading fungus spot that I've been treating with hydrogen peroxide (and will start treating with Betadine tomorrow). I know I should be putting in 1 teaspoon per 1 gallon, but should I be adding 16 spoonfuls at once? Or over a 12 hour period? Please state your source. It's a life or death situation! Thanks in advance!