Mix reverse osmosis water with source water to lower hardness and TDS
In most cases messing with water is a recipe for disaster...
Making water harder takes more effort and the addition of minerals which needs to measured carefully.
Again in most cases many fish will be fine in your source water unless it is very skewed towards one extreme or another..
I keep and raise my GBR in same tap water I keep and raise swordtails in..Both say they prefer different parameters but matching/chasing numbers is not necessary IMO
Now if you want to breed sensitive fish you may need to make water better ,but otherwise I would not ..