Well, moving fish from one environment to another will always have some element of stress to them, but..........
There are chemical alternatives such as Melafix or Stress coat. Doing it with minimal light helps too. Try puncturing little holes in the bag when putting it in the new aquarium. Put some java moss in the bag with the fish, this gives it a place to hide and gives it a level of comfort. When releasing the fish, do not pour out, simply open a hole big enough and allow the fish to exit at own will. Feed the other fish when finally releasing the newcomer, this will keep the others distracted while he gets used to the new tank. Also, make sure that the new tank has a lot of hiding spots for it. Hope this helps.
Don't tap on the glass, the fish will think you are stupid!!