tell me what it is!

Puede que tu medidor de ph esté descalibrado. Controle el ph del agua después de agregar los fertilizantes.

I'm by no means a dwc guy, but I'd look into your room environment a bit closer. In addition to raising your pH, which has been said already, I'd raise the humidity to around 70% at this stage. Another thing you may want to consider is simply raising your lights and/or turning down the intensity. That way you can rule out light stress. I'd start with the easy things before you adjust your feeding strength. 0.8 EC should be fine for a seedling