My guppy has got pop-eye need help !?

i went to a local aquatic shop which has a good reputation and the guy sold me methylene blue today, i noticed the guppy wasn't well last night so i removed it from the tank into 2 gallon container i know it's small but that's all i have at the moment i have an air pump tube going into it to oxygenate the water i don't have a spare filter wot shall i do for treating the guppy in the 2 gallon container also the guy said i should treat my main tank ? all help appreciated if you need any more info just ask