I would start with this:
Add pressurized CO2. Set a steady level, so it is really consistent. This might take a week or two. You want the CO2 to be fairly high, but do not gas the fish.
Decide what parameters you will aim for in the tank water.
Ammonia = 0 ppm
Nitrite = 0 ppm
Nitrate = between 5-20 ppm
...and so on. Set ranges that are suitable to your livestock.
Start a record. Test the tap water at least once a week and record:
Ammonia, NO2, NO3
All other tests you have.
When the water from the city looks the best, do as large a water change as you need to, and think about storing a barrel (perhaps a 50 gallon drum) of that 'best water' to be ready for another water change or 2.
When the water from the city is bad, but you absolutely have to do a water change then use as much RO as you can, and as little city water as you can. Add minerals to the RO water to make it match the GH etc. that you are aiming for.
Look into ways to pre-treat the city water to even it out and get rid of whatever things it has that you do not want in the tank.