Safe way to lower ph

[cpeters5]

Recently bought a Hanna HI 9813-6 to monitor my orchid water supply.

The baseline is rain water, with measurements ~ 6.9pH, 0.015 ms/cm (EC) and 20 ppm (TDS).

After adding a quater teaspoon of NPK 20-20-20 to 1 gallon of rain water, the new measurements become 6.8pH, 0.13 ms/cm and 96 ppm.

Reading Rays response to a similar post, it seems the pH is a little high and TDS is a little low.

What is the safest way to decrease pH and increase TDS?

---------- Post added at 01:54 PM ---------- Previous post was at 01:44 PM ----------

BTW, Our water supply is well water, which is extremely hard. We installed a whole houwe filter system we we moved into this house. The measurement for our tab water is 7.6 ph, 0.35 ms/cm and 251 ppm. As learned from many thread here, I expected my tab water to be too hard for the orchids? I have been using it for about 3 months before I discovered this forum. I switched to rain water about a month ago. Hope some of my plants will recover.