I don't use any additives, just the commercial house washing soaps like talked about in the other thread.
There are some concerns with using well water:
1. Using a pressure washer with 5.6 gpm I heard can make the well pumps burn up as they are not meant to run continuously to fill large tanks.
2. A lot of well water has high amounts of iron (makes things rust) and sometimes high amounts of salt (if close to the coastlines) which can affect your pressure washer.
3. Without a filter after the well pump, there could be sand, dirt and other grit in there that can damage pump packings and the pump.
I always ask if they are on well water or city water. If they are on well water I ask them if their pump will burn up pumping 5.6gpm for an hour or longer, they should know about their pump. Ask if they have filters after the pump or not. Ask more questions to see if the job is viable to do with well water or not.
You might have to bring the tank 1/2 full to start the job and adjust your float so that the level is about 1/2 way in the tank so you don't have their well pump running all the time, just when you need it and slow down work on the job so you are not working so fast like take a lot of breaks so it is easier on their pumps.
This kind of stuff has worked for me in the past, that and a filter before my tank to help keep some of the trash from getting into my tank.