“My tap water is quite alkaline and I have been adjusting it with pH Down which is composed of phosphoric acid. By using it, am I changing the phosphoric acid (P2O5) ratio of my fertilizer solution? Could this result in toxic salt buildup and stress in my plants? When I used rainwater, the stress stopped.”
The has been raising the ppm of salts in the water. This may have thrown the nutrient ratios so out of balance that the plants showed a toxic reaction. This is most likely to happen if the starting water has more than 400ppm dissolved solids, or if it is very alkaline. Then the water should be purified using reverse osmosis. An alternative is to use bottled water.
-Ed Rosenthal The Best of Ask Ed: Your Marijuana Questions Answered