Instruction on the box just for preventative care. Do not follow them it will not help. Salt remedies- every one who had been long enough on this forum learned from Oldfishlady . And a lot of people used those instructions for long period of time ,so it safe. You can use 1tsp/gall for a few days and then increased it to 2 tsp. And you need to finish whole course of the 10 days. I am still confused with your water though ...with salt treatment you need to do daily water changes. I hope you can figure out about water.
Always pre mix salt, make sure it dissolved do not put it directly to his tank.