Runoff ph low?

you need to know what your runoff comes out at under good conditions when nothing is going wrong and the plant is healthy and stays healthy. It is an imprecise measurement due to many factors and is not very representatitve of the substrate. Take all of that x2 for soil. Even soil probes are pretty unreliable for pH. is the plant healthy? do you have a baseline for runoff you are comparing or just assuming what you saw this recent time is a problem? gain familiarity of what is normal before reacting to runoff measurements. If the plant shows symptoms, that's an actionable obsevance. it's good to track this stuff, but it's part of a bigger puzzle as far as how to interpret it.

I'm thinking the same as autoflowersucks said that a runoff of 5.9 is fine. I would not push your input PH any higher. I'd imagine your grow medium will be somewhere between your runoff ph and your input ph.

Nothing wrong with 5.9 I wouldn't use runoff as a means of pH measurement. I would use litmus paper or a digital soil pH Probe

So I don’t actually have the answer for you, but I’m having the same issue..and thought the same thing as you, so that’s what i just did. Was just about to ask if I messed up- then I saw you ask. Hopefully some with good experience chime in soon :) I’ll be watching for your answers. Good luck 🍀