Water in at 6pH ,,runoff at 6.5ph

what type of soil test kit?
Use the slurry method, cup up soil at 1:1 ratio to distilled water wait a couple hours and stick pH meter into that to get your soil pH.

 

If you don’t have any pH down you should consider buying some. phosphoric acid or citric acid is generally what is used.
If you are doing bottle nutrients then use a phosphoric acid ph down.
In a pinch, you can use acetic acid aka white vinegar to reduce your pH but just don’t over do it.

What’s your media made of?

also if you’re already putting in 6 pH water and your soil is 7.8 you might want to consider a different approach.
You need to provide more details on what your media is composed of or what brand.
Hard to say without knowing what your setups like bro

 

First things first, stop watering with distilled water. You are starving your plants to begin with.
As for spring water, it has lime in it and has great effect on buffering up the pH.
Just start using tap water and wasting money on spring water or any bottle water for that matter.

The fact you are already in week 2 flower, you have very little room to play without terribly affecting your yield.

How often do you water? On your next irrigation drop the pH between 5.5 - 6.0 and water with that. Also start feeding your flower nutrients regardless of where your soil pH sits at. It would be stupid to keep giving plain water to a plant that demands enormous amounts of nutrients, especially in flower! Only pH down AFTER adding all nutrients to the reservoir.
Again, you are giving me minimal information about your setup and plants so it is hard to gauge your exact situation