Adding CO2 to lower the pH is about the only reasonable method if the
plants are the item of concern.

Most/many true aquatic plants come from hardwater regions. But the
areas have good CO2 levels.

Plants want CO2, not buffer.

If you want to adjust the hardness/pH, use CO2 gas only to do this.
Some folks with soft water need to add baking soda or GH increasers up
to about 3 degrees but that's about all the plants need.
Most fish are fine as most sold at the LFS's in your area also use tap
water themselves..................

Regards,
Tom Barr