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 marijuana plants? When I used rainwater, the stress stopped.
The ppm of salts in the water has risen as a result of the P2O5. Your nutrient ratios may have been thrown out of balance, revealing a toxic reaction. This will probably occur if the initial water has more than 400 ppm of dissolved solids, or if it is highly alkaline. The water need to be purified through reverse osmosis, or you could just use bottled water.
You should not be using enough pH down solution to cause that much of a difference.
To give you a idea of the increase, just take a gallon of RO water and check the ppm, Then use your pH down solution to adjust the pH to 6.0 and check the ppm.