Vinnie, I LOOOVE these type of threads... it's not controversial at all but ignites many opinions and experiences (7-10+ years ago you would've been lynched upside down with the rope around your nutters... I HATED those responses).
My random, misc thoughts:
When in Hampton Roads, VA in mid 90s, I had discus (and shouldn't have... I was 12 and kept them until leaving for college) and I did old school WCs with buckets and short siphon while refilling with tap. Discus were happy, not shy, ate EVERYTHING... but sucked regarding growth rate. HOWEVER, I gave a few "early stunted" fish to neighborhood buddy (Craig) who's parents used well water only. Within 4 months those discus had a 1-1.5" growth spurt with only 1 50% WC a week... in an overcrowded 20 gallon tamk!!!!!
(plants were growing OUT of the tank as well + even 4 blue rams were well over 3").
-not all well water is created equal... but Craig's well water was like if the fountain of youth and Viagra had a baby.
You're in TN, right? Even if on tap, I'm betting your water has more minerals and harder than mine (and I bet the minerals you have are heavy in the categories of the ones you want if keeping discus...)
-That said, what's the average size discus you have that you first got when they were 2.5-4 months old?
Regardless, I'd love to see you do an experiment that I wish a LOT of us would do --> from same hatch, pick 16 2 month old fry (ALL of same size with only a deviation of +/_ .25mm), split into 2 groups with each having their own 40 gallon, and do your daily WC regime in one but 80% in the other. Do it for 10 months then show: average size, smallest size, then largest (to show dispersion of legnths).