Can you confirm that I am understanding this correctly?

You recommend doing the following:
Drain aquarium tank (50% or whatever)
Add Seachem Prime directly to the aquarium tank.
Take your hose of city water and put it directly into the aquarium tank.

What about other supplements such as Banking Soda for PH adjusment; macro or micro nutrients for the plants?

I have a 125 gal tank. Are you doing this for a 125 gal tank or similar size?
How long have you been doing this without adverse results to fish or plant?

tks