Hey sup[ stinkbudd you have a major problem.

This is what you need to do:

THROW AWAY YOUR MEDIUM!!!!!! Peat moss has a pH around 4.5, and that is a guaranteed KILLER. Take it from me... I've killed an entire crop trying to be cheap and use some leftover peat moss from my outdoor gardening activities for a soil amendment... it's nasty stuff.

Now I'm a bit confused, how does your RO water have pH of 8? That makes no sense! Pure water with atmosheric gases dissolved in it should show a pH just BELOW 7... from the carbonic acid...

The reason your runoff os so low is that the water is reacting chemically with whatever is in the soil, and some of the natural acids in the peat are leaching out into the water as it goes through, and it's just a big mess... you want to start with a medium that is not only already at your desired ph, but is BUFFERED there. I have said this a million times if I've said it once, there's an easy way to guarantee NO ph probs ever, and that's to use a mix that includes a shitload of nice decomposed leaf humus.

On the up side, you are checking your pH correctly using the runoff method.