Doozy is right on there. If you are in soil, stop using the expensive store bought shit and use your tap water. RO or distilled have no mineral content, and thus tend to cause more deficiencies than they help. Trust me on this, MJ is a weed, and will grow just fine in ordinary tap water. And quit obsessing over pH. If your soil pH is right on, it will stay that way throughout the entire grow. The soil will change the pH of whatever goes in, not vice versa. If you water with a solution that has, say a pH of 5, the soil will raise that to whatever the pH of the soil is, hopefully as close to 6.8 as possible. You need to know the pH of the SOIL, not the run off. The water needs to be in contact with the soil for a few minutes in order for the pH to be buffered. Run off is just that, water that just ran thru and wasnt much affected by the soil buffer.
You seem a bit tense. First grow? Relax, this is not all that hard. It can be confusing, since over 75% of all the advise you'll get is BS, or partly so. This is not rocket science. It's simple Botany and Biology 101. If you can grow a fern, you can grow weed.