Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Most houses are plumbed so the outdoor water (usually hose bibs) are not filtered, only indoor water is filtered.
There are certainly exceptions, and I have seen more than one post from aquarium keepers that have no unfiltered water. Might need a plumber to tap into the line before the filter so you can get some water before it is filtered.
Get the water before it is filtered, and test it, post results. It is very rare to have water come out with such extreme readings unless it has been filtered. You might have to keep different fish, if you now have soft water fish.
You are right to worry about the fish adapting to different water. I would do a change like this in small steps over about a month.
Start with 10% water changes 2-3 times per week. Any one water change results in a small shift in the parameters. But it adds up. If the tank needs a bigger change then you will have to blend filtered and unfiltered water until it matches the current parameters. Then you can do as large a water change as needed, since the refill water matches the tank.