Well I tried to match it between 5 - 6 but those color strips suck!
So it probably is off. But after some more research, I realized the hydroponic medium requires a more acidic PH, and the PH affects the uptake of nutrients.
I think this explains why my soil plants are doing much better, because soil can be between 6-7.

But it's still confusing, because if seedling don't need nutrients to begin with, why would having a proper PH be so critical at this stage?

I just ordered a digital PH meter on ebay and a TDS meter.