I would suggest a daily dose of Furan 2 after a 50% or better water change. Also add salt (two tablespoons per 10 gallons) until the fish appears normal. Be sure to do a large daily water change and then redose the Furan 2 and salt of the amount of water in the tank. Did that gash appear overnight? The white stuff is the fish shedding it's slime coat.
Pat