Well find a spot for this byte: Check your AND adjust ph before watering......5.6-6.2 if you are going hydro and 6.2-6.8 if you are growing soil. I'm not familiar with your cubes so they may be doing a number themselves on your seedlings or it may be your water but you'll have to take the issues one at a time till you figure out what the problem is. Many issues mimic each other symptom wise so its a matter of changing things one by one and observing results until you find the problem.

Edit: you are ph'ing ....good. Are you going to grow hydro or in soil once they show their leaves?