Post pics.
Bottled drinking water is no good. It has sodium in it.
Why not use your well water? pH of very soft waters is inconsequential; add calmag if the ppm is below 200 and then the nutes and then test the pH. If your well water is really crappy, distilled water with calmag added to it according to directions on bottle is fine.
Another person adding Epsom salts as a cure-all before diagnosing the actual problem... aaaargh. The symptom you describe does NOT sound like magnesium deficiency!!!!! Phatsesh, tell us what the symptoms of Mg def are? I'm all ears.

It sounds like the plants are pot-bound and suffering the effects of poor soil chemistry. How tall and big around are they, with how many main tops, and how many gallons of soil does each one have, or what diameter pot?