This is hydro you always need to add nutrients. Seedlings should be aboit 150-200 at this point 200-300ppm should be fine. If you use just straight tap your nutrient ratios are gonna be fucked.
Download light meter by my mobile tools dev and should be good at about 20k is right now on an 18/6 light cycle.
Always mix nutrients in the same ratios as recommended unless you understand or have help altering it.
Hydrogaurd needs to be added only at res changes.
I think your issues were caused by light intensity, humidity/temps and improper nutrient ratios.
What is the humidity and temp of the tent?