You can safely go up to .3% or one tablespoon of salt per gallon with no problem, though I would start at one TBSP per 3 gallons and raise if over a few days.
I would try the salt first and see how it goes, don't use in combo with the metro. If there is no change at all in a week, I would change the water out and try another route.
Melafix/pimafix is another idea that may work and the good thing about this is that it would be effective if the root cause is bacterial or fungal. However if the fish won't eat anything, you may not have time to try two different "easy methods", and may want to go straight to harder medicine. If you do decide to use Metro, here's a helpful quote from Dr. Edward Noga which
RD.
had posted a while back
r. Edward J Noga suggests:
1. Bath
a. Add 19mg/gallon and treat for 3 hours. Repeat every other day for 3 treatments.
2. Prolonged Immersion
a. Add 25mg/gallon and treat once daily for a total of 3 times.
b. Add 95mg/gallon and treat every other day for three days.
3. via feed
25 mg/Kg of body weight/day for 5 to 10 days, or 100 mg/kg of body weight for 3 days. Retreat once if required.
I would do the prolonged immersion dose in the hospital tank. Metro stays in the water for 8 hours from my understanding, so if you dose in the morning you could do water changes at night, and then re-dose the next day, to keep the water very clean and oxygenated during the treatment.
