High DonCostro,
The roots do not get enough oxygen. You need to let the soil dry out before watering again. Your pH is perfect and the soil contains more then enough nutrients for this stage!
So let the top 5cm from your soil dry out before you give water again. And try to water around 10cm away from your plant so that the roots have to search for the water and also have acces to oxygen!
Happy Growing! 😋