I would not put it back in the main tank.
How long has the fish been on metro? Feeding fish with hex can be tough, as it inflames their throats and makes it hard to swallow. Whenever I had to deal with it, I did 10 days of metro powder at 1g per 40 gallons with 100% wc between doses. Additionally, 10 days of metro dosed pellets or frozen at 1g metro powder per 1lb food when they started eating again. A gram of metro is roughly a heaping tsp of the pure powder. I would use 2 spoonfuls of the Metroplex, as this will get you pretty close to 250ml per 10 gal.