Please don't add another goldfish!
Plants should help make some hiding places, hardy live plants or smooth leaves in plastic plants or silk, someplace for him to feel secure. Do some research on tank mates, more goldfish require much more space and really strong filtration.
If you have a hang on the back/ HOB filter, you can get a piece of filter media replacement pad and tuck a piece appx. the size of your filter pad into the out flow into the tank, it helps diffuse the flow of water.
Then maybe after the tank is established and you see if your goldie is doing well and adjusting to new life in a bigger space, you can see what steps to take next.
No pH test needed right now. You are using the same water out of your tap each pwc right? Should be the same more or less, the same pH.
Adding extra bubbler is good to keep oxygen up.