we are from an area of "rock hard" water... so the bulk of my tanks I use a dedicated RO unit... but I'm kind of curious on what happens as naturally hard water is softened with organic materials... I know the calcium acts like a buffer... & we live in an area where the hardness in our water is calcium... but I think in our state ( in the "iron range" ) for example, that the hardness is likely from iron... so that water wouldn't have that same buffering effect... also if you were only doing 10% water changes weekly and had a lot of organics in your tank, I'd think you could overcome the buffering effect, & see a Ph swing downward... I do add well water to a hard water fish tank, & to my Cherry shrimp tank, but the fish tank is lined across the whole back of the tank, with hanging pots of Pothos vines... each pot is filled with Ceramic bio media, with peat beads mixed in ( in effect, to soften the water around the plant )... my shrimp tank, is only a 10 gallon, but gets a couple Almond leaves as fast as they are decomposed or consumed...
so really, I'm curious is the fish need the minerals or the Ph???