Take a picture of him while he's in the 5 gallon tank.
I don't know why he would b stressed UNLESS the water in the tank is off.
You've obviously taken the time to decorate it nice, but you NEED to test the water to make sure it's safe.
An aquarium test kit is essential if you want to keep your fish healthy.
Get a kit that checks for....
BTW, are you using a water conditioner? If so what brand?
I wouldn't add anything else to the tank.